Better volumetric test

[PlethoraChutney]

Hello! Thank you for MDAnalysis! This PR changes how MRC files are checked before being loaded as a 3D grid. Rather than use the is_volume() method from mrcfile we instead use a simpler shape check.

The reason for this is that mrcfile returns False if the ISPG header field of a 3D MRC file is zero. While this is correct according to the spec, most cryoEM software packages save a 0 in this field for a variety of reasons. Because of this unwritten deviation from spec, most packages also read in files with a 0 space group as 3D Volumes anyway, but GridDataFormats does not.

Despite the fact that this is formally correct, I'm proposing that MDAnalysis should follow the behavior that users likely expect from their experience with other packages.

This PR fails three tests, but on my machine at least the current master branch also fails these same tests.

• gridData/tests/test_grid.py::TestGrid::test_load_wrong_fileformat[ccp4]
• gridData/tests/test_grid.py::TestGrid::test_load_wrong_fileformat[plt]
• gridData/tests/test_grid.py::TestGrid::test_load_wrong_fileformat[dx]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.70%. Comparing base (abd0feb) to head (f391ee3).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #149   +/-   ##
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  Coverage   87.70%   87.70%           
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  Files           5        5           
  Lines         748      748           
  Branches       97       97           
=======================================
  Hits          656      656           
  Misses         56       56           
  Partials       36       36           

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