Add QPI features

[PinkShnack]

Description

As described in #192, we should put optional QPI-related features into dclab so that .rtdc files can handle them.

See list of features in #192.

To-do list

• [ ] Create the below features in dclab (calc if needed) with correct dtype
• [ ] Add test qpi dataset to dclab
• [ ] Add tests for each feature
• [ ] Test reading and writing of qpi features to .rtdc file

Tests

• [ ] qpi_oah
• [ ] qpi_oah_bg
• [ ] qpi_pha
• [ ] qpi_amp
• [ ] qpi_dm
• [ ] qpi_ri
• [ ] qpi_pha_int

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (3d9d0e2) 90.96% compared to head (92be592) 90.96%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #225 +/- ## ======================================= Coverage 90.96% 90.96% ======================================= Files 97 97 Lines 7070 7070 ======================================= Hits 6431 6431 Misses 639 639 ```

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[PinkShnack]

Closing, to be done at a later point (see comment in #192)