(feat): elem_mul 64 bit upcast option

[ilan-gold]

• Closes elem_mul in variance calculation should use float64 casting #3127
• Tests included or not required because:

• Release notes not necessary because:

[ilan-gold]

I'm putting this in scanpy2.0 because this setting should be turned on then

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.61%. Comparing base (8b0c3f6) to head (f94547b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3392      +/-   ##
==========================================
+ Coverage   76.59%   76.61%   +0.01%     
==========================================
  Files         111      111              
  Lines       12874    12884      +10     
==========================================
+ Hits         9861     9871      +10     
  Misses       3013     3013              

Files with missing lines	Coverage Δ
src/scanpy/_settings.py	90.51% <100.00%> (+0.30%)	⬆️
src/scanpy/_utils/__init__.py	76.38% <100.00%> (+0.09%)	⬆️

[flying-sheep]

So this will just use 64 bits for computation and downcast again after?