BF+ENH: Fixes to DICOM scaling, make frame filtering explicit

[moloney]

Fixes how we handle DICOM scaling, particularly for Philips and multi-frame files. For Philips data scale factors without defined units should be avoided, and instead a private tag should be used to make image intensities comparable across series. For multi-frame DICOM, it is possible to have different scale factors (potentially coming from different tags) per-frame. We also prefer scale factors from a RealWorldValueMapping provided they have defined units.

The base Wrapper class now has a few new attributes and methods to support this functionality. In particular an attribute scale_factors that provides an array of slope/intercept pairs, and a method get_unscaled_data that will return the reordered/reshaped data but without the scaling applied. A vendor attribute was also added to better support vendor-specific implementation details.

For the MultiFrameWrapper I also added an attribute frame_order which exposes the order used to sort the frames, and use this to return the scale_factors in sorted order. While implementing this I kept bumping into issues due to the (implicit) frame filtering that was happening in the image_shape property, so I made this filtering explicit and configurable and moved it into the class initialization.

[codecov]

Codecov Report

Attention: Patch coverage is 94.45844% with 22 lines in your changes missing coverage. Please review.

Project coverage is 95.34%. Comparing base (33c6721) to head (f0264ab).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
nibabel/nicom/dicomwrappers.py	91.53%	8 Missing and 8 partials ⚠️
nibabel/nicom/tests/test_dicomwrappers.py	97.96%	2 Missing and 2 partials ⚠️
nibabel/nicom/utils.py	81.81%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1342      +/-   ##
==========================================
- Coverage   95.35%   95.34%   -0.01%     
==========================================
  Files         207      207              
  Lines       29238    29481     +243     
  Branches     4905     4981      +76     
==========================================
+ Hits        27881    28110     +229     
- Misses        925      930       +5     
- Partials      432      441       +9     

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[moloney]

Also, this allows us to get data out of multi-frame files with more than one StackID. By default the lowest StackID will be used and a warning will be issued, but a user can pass in a frame filter that explicitly selects the StackID. I haven't run into this situation yet but it is mentioned here.

[effigies]

@moloney Thanks for this (and thanks for your patience)!