Deprecate ParmedConverter taking Timestep objects

[lilyminium]

Fixes #
Relates to #3031

Changes made in this Pull Request:

• Fixed ParmedConverter reading Timestep objects, at least the AttributeErrors
• Deprecated ParmedConverter reading Timestep objects

PR Checklist

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

[pep8speaks]

Hello @lilyminium! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-04-03 18:33:17 UTC

[codecov]

Codecov Report

Merging #3172 (2d5e901) into master (c6f1a5a) will increase coverage by 0.86%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3172      +/-   ##
==========================================
+ Coverage   90.96%   91.82%   +0.86%     
==========================================
  Files         162      167       +5     
  Lines       22196    22702     +506     
  Branches     3201     3201              
==========================================
+ Hits        20191    20847     +656     
- Misses       1382     1771     +389     
+ Partials      623       84     -539     

Impacted Files	Coverage Δ
package/MDAnalysis/coordinates/ParmEd.py	90.55% <100.00%> (+2.28%)	⬆️
package/MDAnalysis/topology/tpr/obj.py	96.82% <0.00%> (-3.18%)	⬇️
...onality_reduction/DimensionalityReductionMethod.py	97.14% <0.00%> (-2.86%)	⬇️
package/MDAnalysis/analysis/legacy/x3dna.py	0.00% <0.00%> (ø)
package/MDAnalysis/due.py	56.09% <0.00%> (ø)
package/MDAnalysis/tests/__init__.py	100.00% <0.00%> (ø)
package/MDAnalysis/analysis/legacy/__init__.py	0.00% <0.00%> (ø)
package/MDAnalysis/tests/datafiles.py	31.25% <0.00%> (ø)
package/MDAnalysis/core/selection.py	99.52% <0.00%> (+<0.01%)	⬆️
...sis/analysis/encore/clustering/ClusteringMethod.py	96.96% <0.00%> (+0.04%)	⬆️
... and 86 more

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[IAlibay]

Couple of minor things

[IAlibay]

Still gotta live that py2.7 life :(