Implement and debug DDRTree based methods

[Sichao25]

• Implement order_cells function which calculates pseudo time and order cells.
• Implement construct_velocity_tree function which integrates pseudo time and velocity to construct a directed graph.
• Debug time_series, which uses matrix calculation to calculate directed graph.
• Implement corresponding plotting functions

[codecov-commenter]

Codecov Report

Merging #564 (6a2cc7a) into master (e5e9017) will decrease coverage by 0.05%.
The diff coverage is 9.78%.

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@@            Coverage Diff             @@
##           master     #564      +/-   ##
==========================================
- Coverage   22.51%   22.46%   -0.05%     
==========================================
  Files         165      165              
  Lines       26991    27339     +348     
==========================================
+ Hits         6077     6142      +65     
- Misses      20914    21197     +283     

Files	Coverage Δ
dynamo/plot/__init__.py	100.00% <100.00%> (ø)
dynamo/plot/utils.py	21.10% <ø> (ø)
dynamo/tools/__init__.py	100.00% <100.00%> (ø)
dynamo/tools/DDRTree_py.py	18.07% <0.00%> (ø)
dynamo/tools/time_series.py	28.57% <0.00%> (+28.57%)	⬆️
dynamo/plot/pseudotime.py	14.41% <12.98%> (+14.41%)	⬆️
dynamo/tools/construct_velocity_tree.py	7.81% <9.09%> (+7.81%)	⬆️
dynamo/tools/pseudotime.py	9.09% <7.10%> (+9.09%)	⬆️

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