Fix CoordinateSource (allow rectangular micrographs)

[j-c-c]

Resolves #1253 by adding a private Image._load method that returns a numpy array of data and pixel_size and a public Image.load that returns an Image instance. The private method is used by CoordinateSource to load data from mrc/star files, which allows for rectangular micrographs.

Also, replaces a raise in Image._vx_array_to_size() with a log message to be more permissive of badly formatted headers.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.61%. Comparing base (4b85dd9) to head (6e86d91).
Report is 7 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1254      +/-   ##
===========================================
+ Coverage    90.59%   90.61%   +0.01%     
===========================================
  Files          132      132              
  Lines        14136    14143       +7     
===========================================
+ Hits         12807    12815       +8     
+ Misses        1329     1328       -1     

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[janden]

Looks good. Just a name change suggestion.