Removing deprecated reader code for v1.0

[IAlibay]

Partly addresses #2327 and #2443

As far as I am aware, the NetCDF writer is the only part of the reader/writers that has code set for removal by v1.0.0, please let me know if this isn't the case.

Changes made in this Pull Request:

• Removes support for the degrees unit for cell_angles in the AMBER NetCDF reader.

Note: this does not completely fix the NetCDF reader/writer (scale_factor writer support is needed, and we really should switch the writing of velocities from unscaled to the same factor used by the AMBER MD engines use [see #2327]). However, assuming a non API complete v1.0.0, further fixes can probably be done by the next release.

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[tylerjereddy]

LGTM if CI passes. The deprecation was issued in August 2019---do we have a timeline for deprecation->removal workflow established (i.e., two years? two releases minimum? etc.).

[codecov]

Codecov Report

Merging #2482 into develop will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #2482      +/-   ##
===========================================
- Coverage    90.25%   90.25%   -0.01%     
===========================================
  Files          179      179              
  Lines        23281    23277       -4     
  Branches      3009     3008       -1     
===========================================
- Hits         21012    21008       -4     
  Misses        1662     1662              
  Partials       607      607

Impacted Files	Coverage Δ
package/MDAnalysis/coordinates/TRJ.py	94.81% <100%> (-0.06%)	⬇️

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[IAlibay]

LGTM if CI passes. The deprecation was issued in August 2019---do we have a timeline for deprecation->removal workflow established (i.e., two years? two releases minimum? etc.).

Thanks for the review!

With regards to the deprecation->removal workflow, I am currently going through all the things that need to be removed for v1.0.0, and there doesn't seem to be much consistency. For the most part a minimum of two versions have been released before removal (this particular case has the shortest deprecation->removal time, deprecated in 0.20 -> 0.20.1 -> removal 1.0).

That being said, if the consensus is that this removal is too soon, I don't mind shelving it for a later date.