Properly address units in PintPulsar position

[vhaasteren]

This fixes #363

By using astropy.units to convert to units.rad, it doesn't matter what units PINT returns, the position is read in radians.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 88.37%. Comparing base (5c1e455) to head (c58f840).
Report is 22 commits behind head on dev.

❗ Current head c58f840 differs from pull request most recent head 7b7eb22. Consider uploading reports for the commit 7b7eb22 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #370      +/-   ##
==========================================
- Coverage   88.44%   88.37%   -0.07%     
==========================================
  Files          13       13              
  Lines        3037     3036       -1     
==========================================
- Hits         2686     2683       -3     
- Misses        351      353       +2     

Files	Coverage Δ
enterprise/pulsar.py	91.62% <50.00%> (-0.46%)	⬇️

... and 1 file with indirect coverage changes

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[vhaasteren]

Note that the lines not covered by tests were already not covered.

I think it's actually worthwhile to include a NG 5yr dataset (say, B1855+09) in the test data (under tests/data/), so we can load a quick PINT pulsar with RAJ/DECJ parameters and also run those tests. PINT also includes that pulsar in its test data. Having a pulsar with RAJ/DECJ that is not 1713 is useful to have for other unit tests that still need to be implemented as wel. Right now we only have 1713.Sep.T2.par/tim

[vhaasteren]

Also note that the designmatrix and model parameter units are not addressed by this PR. That is a separate issue that should be addressed in a more comprehensive PR

[vhaasteren]

This one depends on #382 for the added test to pass. So @tcromartie, this last test I added was missing (uncovered lines). If it had been there from the start, your PR #382 would have never been happening.

Other than that, this PR is ready