Fully randomized grids; Lazy-loading model dictionary

[chenyangkang]

Related: #59; #69

1. Add a option for completely randomized grids generation (compared to equal division of the 90 degree angle).
2. Implement Lazy-loading model dictionary for saving memory; Save ensmebles of models to disk when finish training, and loaded it when used for prediction
3. Update init parameters in AdaSTEM classes, STEM classes, and SphereAdaSTEM classes.
4. Update lazy loading documentation & example notebooks.
5. Add related pytests.

[codecov]

Codecov Report

Attention: Patch coverage is 93.05302% with 38 lines in your changes missing coverage. Please review.

Project coverage is 89.99%. Comparing base (32f80a7) to head (2f6dc8f).
Report is 32 commits behind head on main.

Files with missing lines	Patch %	Lines
stemflow/utils/lazyloading.py	80.00%	28 Missing ⚠️
stemflow/model/AdaSTEM.py	94.69%	6 Missing ⚠️
stemflow/utils/validation.py	70.00%	3 Missing ⚠️
stemflow/utils/plot_gif.py	98.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #69      +/-   ##
==========================================
+ Coverage   87.05%   89.99%   +2.94%     
==========================================
  Files          31       34       +3     
  Lines        2139     2508     +369     
==========================================
+ Hits         1862     2257     +395     
+ Misses        277      251      -26     

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