Allow exporting of plots with a categorical size attribute.

[Carifio24]

While working on adding the perspective option, I noticed that 3d plots will not export if the size attribute is categorical, despite the fact that this is allowed in glue. This is due to the subtraction in the numerator here being undefined (since the first array is non-numeric).

This PR fixes this issue by using ensure_numerical on the categorical data. This is the same technique used in the 2d scatter layer artist, as well as in the 3d scatter layer artist (it doesn't make a call to ensure_numerical there, but it performs the same operation here).

This shouldn't be an issue for the 2d scatter export (the 2d scatter viewer restricts the possible size attributes to numerical ones, which the 3d viewer doesn't), but I've added the same operation there for good measure.

[codecov[bot]]

Codecov Report

Merging #14 (128045e) into main (7d3eddf) will decrease coverage by 0.36%. The diff coverage is 33.33%.

@@            Coverage Diff             @@
##             main      #14      +/-   ##
==========================================
- Coverage   86.11%   85.75%   -0.37%     
==========================================
  Files           8        8              
  Lines         396      400       +4     
==========================================
+ Hits          341      343       +2     
- Misses         55       57       +2     

Impacted Files	Coverage Δ
glue_plotly/html_exporters/scatter2d.py	81.55% <33.33%> (-0.63%)	:arrow_down:
glue_plotly/html_exporters/scatter3d.py	76.40% <33.33%> (-0.61%)	:arrow_down:

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