OER/COER implementation

[JNevrly]

This is a support for OER/COER codec as defined in ITU-T X.696 (08/2015). Covers the specification EXCEPT sections:

• 23 Encoding of values of the internationalized resource reference type - did not find matching support inside Pycrate)
• 24 Encoding of values of the relative internationalized resource reference type - did not find matching support inside Pycrate)
• 25 Encoding of values of the Embedded-PDV type - left it on internal Pycrate's handling using Open type
• 26 Encoding of values of the External type - left it on internal Pycrate's handling using Open type
• 29 Encoding of values of the time type - did not find matching support inside Pycrate.

Notes

• The implementation provides both to/from_oer and to/from_coer methods, but the encoding is COER only (there was no significant performance gain using OER-allowed possibilies). The *_coer methods are therefore currently just wrappers for the *_oer methods.
• I tried to match the code style to the Pycrate code-style (being aware that it deviates from PEP8 in many points). As OER/COER stands somewhere between PER and BER, I reused relevant PER/BER sections of the code. The code readability is therefore maintainer-oriented ;).
• The *_ws methods are implemented, but as I don't quite understand the motivation for these methods in the first place, I'm not sure if the implementation is sufficient...

Testing

• Simple types were tested against specification, constructed types were tested against ASN.1 Playground.
• Integration test was done against the only COER sources available for me (valid ITS Security messages). This testing is limited by issues #100, #109, but the codec parts seem to be working alright.

TODO's

• support for sections mentioned above (when respective Pycrate support is implemented)
• Check of current Embedded-PDV and External type encoding implementation (missing suitable reference material)
• Addding OER/COER OID values into the Pycrates codec dictionary - I did not really find a part in PyCrate that would have that dictionary. As I understand, ENCODED BY is anyways currently ignored.

[p1-bmu]

I started to review the PR: this looks good. Thank you very much for your efforts !

Here are few questions:

• I've seen you introduced new functions in pycrate_core/utils_py3.py related to REAL OER encoding / decoding. Does it mean it is not intended to work with Python 2.7 (at least for REAL object) ?
• In some case, you are calling the PER codec (at least for BOOL): is it intended to be the aligned or unaligned variant ?

As the code is well constructed and independent from the rest, I have merged it: this will enable to continue working on fixes and more support around this OER codec and the specs that make use of it.

[JNevrly]

Thanks for the merge!

• I've seen you introduced new functions in pycrate_core/utils_py3.py related to REAL OER encoding / decoding. Does it mean it is not intended to work with Python 2.7 (at least for REAL object) ?

No, I just haven't understood the utils* structure. There is no Python3-related code in those utilities, I suppose it should go to utils.py. I'll do a PR with fix for that. However, although I have tried to avoid constructs that would not work in Python2.7, I have not tested it in Python2.7. I'll try to check it.

• In some case, you are calling the PER codec (at least for BOOL): is it intended to be the aligned or unaligned variant ?

IIRC the only place I called PER functions is in the NULL object and that should be alignment-independent, if I understood that correctly. If I did call it in some situation where it is alignment-related, please let me know.