Adding Appending Functionality method to WriterBase

[yuxuanzhuang]

Fixes #3861

Changes made in this Pull Request:

• add new SingleFrameWriterBase
• add new flag append for appending trajectory to WriterBase
• add a check for the number of frames written by WriterBase.
• make sure write is not overwritten by Writers (they only modified _write_next_frame)

TODO:

• add tests for writers that are not covered by BaseWriterTest

PR Checklist

• [x] Tests?
• [] Docs?
• [] CHANGELOG updated?
• [x] Issue raised/referenced?

[codecov[bot]]

Codecov Report

Base: 94.31% // Head: 94.34% // Increases project coverage by +0.03% :tada:

Coverage data is based on head (523a46f) compared to base (35248e8). Patch coverage: 95.50% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3863      +/-   ##
===========================================
+ Coverage    94.31%   94.34%   +0.03%     
===========================================
  Files          193      193              
  Lines        25024    25068      +44     
  Branches      3374     3376       +2     
===========================================
+ Hits         23601    23651      +50     
+ Misses        1374     1368       -6     
  Partials        49       49              

Impacted Files	Coverage Δ
package/MDAnalysis/coordinates/XYZ.py	92.05% <75.00%> (-0.76%)	:arrow_down:
package/MDAnalysis/coordinates/PDB.py	95.62% <78.57%> (+0.48%)	:arrow_up:
package/MDAnalysis/coordinates/CRD.py	93.20% <100.00%> (+1.04%)	:arrow_up:
package/MDAnalysis/coordinates/DCD.py	100.00% <100.00%> (ø)
package/MDAnalysis/coordinates/FHIAIMS.py	95.09% <100.00%> (ø)
package/MDAnalysis/coordinates/GRO.py	96.66% <100.00%> (ø)
package/MDAnalysis/coordinates/H5MD.py	97.60% <100.00%> (-0.01%)	:arrow_down:
package/MDAnalysis/coordinates/MOL2.py	97.67% <100.00%> (ø)
package/MDAnalysis/coordinates/NAMDBIN.py	100.00% <100.00%> (ø)
package/MDAnalysis/coordinates/PDBQT.py	87.75% <100.00%> (+0.12%)	:arrow_up:
... and 12 more

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