Make compatible with xrayvision.visibility.Visibilities

[paolomassa]

Do not merge before TCDSolar/xrayvision/pull/67

Make the imaging demo compatible with the new xrayvision.visibility.Visibilities class. To be merged after #67.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 72.57%. Comparing base (da09731) to head (54d4e07).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
stixpy/imaging/em.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #123   +/-   ##
=======================================
  Coverage   72.57%   72.57%           
=======================================
  Files          31       31           
  Lines        1856     1856           
=======================================
  Hits         1347     1347           
  Misses        509      509           

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[samaloney]

So I think the doc build fails are because the PR on xrayvision wasn't merged. I proded the doc_build test if that passses will merge.