Allow specifying visual attributes when creating subset group

[Carifio24]

This PR allows specifying visual attributes when creating a new subset group, either via the SubsetGroup initializer or the DataCollection::new_subset_group method.

I don't know that this adds much, if any, utility to the standard glue workflow - the intended use case here is for dashboard-style applications like Cosmic Data Stories. In a setting where the application is doing the styling, the delay between creating the new subset group and modifying the styling is occasionally visible, which this PR is looking to solve.

[codecov]

Codecov Report

Merging #2297 (4b7c005) into main (8bc18e3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2297   +/-   ##
=======================================
  Coverage   88.15%   88.15%           
=======================================
  Files         247      247           
  Lines       23325    23327    +2     
=======================================
+ Hits        20563    20565    +2     
  Misses       2762     2762           

Impacted Files	Coverage Δ
glue/core/data_collection.py	90.33% <100.00%> (ø)
glue/core/subset_group.py	99.15% <100.00%> (+<0.01%)	⬆️
glue/core/visual.py	87.38% <100.00%> (+0.11%)	⬆️

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[astrofrog]

Looks good! Could you add a test though?

[Carifio24]

@astrofrog I've added two tests - one to check that the correct default values are applied in new_subset_group, and another with an explicit set of visual attributes. Let me know if there are any other sorts of tests that you think would be good to add.

I've also changed the default value for markersize in the SubsetGroup constructor to 7, rather than 7.5, since the VisualAttributes setter converts it into a int anyways. (I had originally put in 7.5 to match the existing value that was assigned - the default value for markersize in VisualAttributes is 3, and the current code for new_subset_group multiplies the markersize by 2.5).

[astrofrog]

Looks good, thanks!