Fix 3MF color export and partial support for color import

[jdegenstein]

This PR reworks the current 3MF color export and color import to use material groups rather than the current approach of vertex specific colors. This PR allows for one to export objects from build123d with a color to a 3MF and then re-import that 3MF file with color preserved.

Known Limitations:

• Removes vertex coloring from current implementation
• Does not currently support 3MF color groups, which may be possible to support
• One color per object

from build123d import *
a=Solid.make_box(1,2,3)
b=Solid.make_box(3,3,1)
a.color = Color(0.137, 0.306, 0.439)  # Tealish
b.color = Color(0.980, 0.973, 0.749)  # Goldish
exporter = Mesher(unit=Unit.CM)
exporter.add_shape([a,b])
exporter.write("ab_dual_color.3mf")

And importing:

importer = Mesher()
objects = importer.read("ab_dual_color.3mf")

Which can then be shown e.g. show(objects)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.98%. Comparing base (e4ce1d8) to head (1c3f895).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #594   +/-   ##
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  Coverage   94.98%   94.98%           
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  Files          25       25           
  Lines        7999     7984   -15     
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- Hits         7598     7584   -14     
+ Misses        401      400    -1     

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