Pixel Size Attributes

[garrettwrong]

Closes #1152 .

Adds pixel_size attribute to Volume and tests saving/loading default and non-default values.
Adds pixel_size attribute to Image and tests saving/loading default and non-default values.
Tests downsampling with default and non-default values.
Partially refactors Image tests to actually use fixtures (faster) and scopes them.

[garrettwrong]

At this point I will move onto trying to have ImageSource serve up Images with pixel_size populated, and the associated tests for that. It wasn't really part of the original issue, but might-as-well while I'm in here.

[garrettwrong]

Some more work done here. Added pixel_size to synthetic volume generators, which should flow through from a user providing into simulation, or allow them to generate their own volumes with the attr.

After thinking about some options for dealing with filters, it seems like the only tricky case was Simulation, and its only if a user explicitly provides pixel_size and brings their own filters. I've added a simple private method to check this case and raise on mismatch. I believe all other cases should work fine with the existing filter scale approach; unless I've missed one.

Still need to add some more tests.

[garrettwrong]

Confirmed the pixel_size is being picked up in ChimeraX when saved from ASPIRE.

[j-c-c]

This looks great. Just a few things.

[j-c-c]

Looks good!

[janden]

Looks great! Thanks for tracking down the pixel_sizes wherever they've managed to hide themselves. Just a few questions here.

[garrettwrong]

Rebased to resolve minor downsample merge ocnflicts.