DensMAP support

[AlePur]

I added an optional boolean parameter to scanpy.tl.umap for DensMAP. Since umap 0.5, simplicial_set_embedding takes densmap and densmap_kwds parameters, and tools/_umap is using this function, so DensMAP support is easily implemented.

[codecov[bot]]

Codecov Report

Merging #2684 (7b6a875) into master (42e3c2a) will decrease coverage by 0.16%. Report is 1 commits behind head on master. The diff coverage is 83.33%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2684      +/-   ##
==========================================
- Coverage   71.98%   71.83%   -0.16%     
==========================================
  Files         108      108              
  Lines       11920    11921       +1     
==========================================
- Hits         8581     8563      -18     
- Misses       3339     3358      +19     

Files	Coverage Δ
scanpy/tools/_umap.py	72.05% <83.33%> (+0.41%)	:arrow_up:

... and 1 file with indirect coverage changes

[flying-sheep]

I see, densmap

Hmm, I think that method='densmap' and method_kwds={...} would be a better API for us (which would then be translated into densmap=True, densmap_kwds=method_kwds)

This also needs tests and a release note.

Also we probably should just remove the umap 0.4 compatibility code, what do you think @ivirshup?