Compute initial states

[michalk8]

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Title

See below.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• New feature (non-breaking change which adds functionality)

Description

Update docs, change default for cr.tl.transiton_matrix add _compute_initial_states for GPCCA.

How has this been tested?

Yes, new tests for the new functions.

Closes

#401
#402
#403

[codecov]

Codecov Report

Merging #404 into master will increase coverage by 0.06%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #404      +/-   ##
==========================================
+ Coverage   80.07%   80.13%   +0.06%     
==========================================
  Files          49       49              
  Lines        6248     6267      +19     
==========================================
+ Hits         5003     5022      +19     
  Misses       1245     1245              

Impacted Files	Coverage Δ
cellrank/tl/_transition_matrix.py	96.15% <ø> (+15.38%)	⬆️
cellrank/tl/_utils.py	82.54% <ø> (ø)
cellrank/ul/_docs.py	100.00% <ø> (ø)
cellrank/tl/estimators/_gppca.py	86.41% <80.00%> (-0.37%)	⬇️

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