increase coverage to 100% (test ALL AMBER files and all of __init__)

[orbeckst]

• fix #49
• test all untested AMBER data sets (custom tests (with hacked accessors) for the AMBER BACE example and invalidfiles)
• test everything in __init__.py (Bunch and __version__)

[codecov[bot]]

Codecov Report

Merging #74 (31114d8) into master (6eda6eb) will increase coverage by 9.21%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #74      +/-   ##
==========================================
+ Coverage   90.13%   99.34%   +9.21%     
==========================================
  Files          11       11              
  Lines         152      152              
  Branches       19       19              
==========================================
+ Hits          137      151      +14     
+ Misses         15        1      -14     

Flag	Coverage Δ
unittests	99.34% <ø> (+9.21%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/alchemtest/__init__.py	94.11% <0.00%> (+23.52%)	:arrow_up:
src/alchemtest/amber/access.py	100.00% <0.00%> (+37.03%)	:arrow_up:

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[orbeckst]

@xiki-tempula I added tests for all current AMBER files.

The ones for invalid files will change with PR #71 (and my hack can then be removed) but at least we now have a clear baseline. It also documents the non-API behavior of the BACE example.