Extensions to VariableLength: support arrays whose length is determined by another field

[ddasilva]

This change updates the VariableLength class to support variable length fields where the length is determined by another field in the packet. For example, you could have fields data1_len which sets the length of the data array, and then data2_len which follows it and sets the length of the data2 array. I have been told that some instruments generate these types of packets, which were currently unsupported by the array_shape="expand" method.

I added a new test for this and all the previous tests pass. I updated the user's guide for variable length packets and added to the changelog.

   import ccsdspy
   from ccsdspy import PacketField, PacketArray

    pkt = ccsdspy.VariableLength([
         PacketField(
              name='SHCOARSE',
              data_type='uint',
              bit_length=32
         ),
         PacketField(
              name='data_len',
              data_type='uint',
              bit_length=8,
         ),  
         PacketArray(
              name="data",
              data_type="uint",
              bit_length=16,
              array_shape="data_len",  # links data to data_len
         ),
         PacketField(
              name="checksum",
              data_type="uint",
              bit_length=16
         ),
    ])

[codecov-commenter]

Codecov Report

Patch coverage: 97.80% and project coverage change: +0.10 :tada:

Comparison is base (e15f2bf) 94.82% compared to head (4cb0354) 94.92%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #57 +/- ## ========================================== + Coverage 94.82% 94.92% +0.10% ========================================== Files 6 6 Lines 464 493 +29 ========================================== + Hits 440 468 +28 - Misses 24 25 +1 ``` | [Impacted Files](https://codecov.io/gh/ddasilva/ccsdspy/pull/57?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Daniel+da+Silva) | Coverage Δ | | |---|---|---| | [ccsdspy/packet\_fields.py](https://codecov.io/gh/ddasilva/ccsdspy/pull/57?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Daniel+da+Silva#diff-Y2NzZHNweS9wYWNrZXRfZmllbGRzLnB5) | `96.61% <80.00%> (-1.61%)` | :arrow_down: | | [ccsdspy/packet\_types.py](https://codecov.io/gh/ddasilva/ccsdspy/pull/57?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Daniel+da+Silva#diff-Y2NzZHNweS9wYWNrZXRfdHlwZXMucHk=) | `95.71% <83.33%> (-0.61%)` | :arrow_down: | | [ccsdspy/decode.py](https://codecov.io/gh/ddasilva/ccsdspy/pull/57?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Daniel+da+Silva#diff-Y2NzZHNweS9kZWNvZGUucHk=) | `95.43% <100.00%> (+1.14%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Daniel+da+Silva). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Daniel+da+Silva)

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[ddasilva]

@ehsteve Let me know what you think of this at a high level. There's a lot of dense code added, no pressure to review it

[ddasilva]

Closes #58

[ddasilva]

Build of Updated documentation (hidden from public): https://ccsdspy.readthedocs.io/en/var-length-prev-field/user-guide/variablelength.html

[ehsteve]

Hey @ddasilva maybe I'm not understanding something but given that the packet length is already in the ccsds header, isn't it always possible to work out the length of a variable field with this new functionality. Seems to me like any data length field is redundant unless you are relaxing the requirement on having only one expanding field.

[ddasilva]

That is correct, you have multiple variable length fields per packet.

[ddasilva]

And there is a unit test that does just this!

[ddasilva]

I updated the user guide to show the example having two reference-based variable fields, and added a statement to the user guide about how there is not a limit to the number of these reference-based variable fields, but there is for expanding fields.

https://ccsdspy.readthedocs.io/en/var-length-prev-field/user-guide/variablelength.html

[ddasilva]

@ehsteve I merged this because several weeks went by and I assumed you didn't have any comments on this. If you have comments, let me know and we can go back to discuss