Add NAMD IDWS dataset, lambda 1 -> 0, with interruptions and restarts

[ttjoseph]

@orbeckst This is the new test dataset to go along with my/@jhenin's changes to the alchemlyb NAMD parser

[codecov]

Codecov Report

Merging #57 (e518b7c) into master (3090d01) will increase coverage by 0.54%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
+ Coverage   88.88%   89.43%   +0.54%     
==========================================
  Files           9        9              
  Lines         135      142       +7     
==========================================
+ Hits          120      127       +7     
  Misses         15       15              

Flag	Coverage Δ
unittests	89.43% <100.00%> (+0.54%)	⬆️

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Impacted Files	Coverage Δ
src/alchemtest/namd/__init__.py	100.00% <100.00%> (ø)
src/alchemtest/namd/access.py	100.00% <100.00%> (ø)

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[orbeckst]

Please just add the data set to the docs in namd.rst and this is ready to go!

[orbeckst]

awesome, thank you!