Add `Structure.sublattices` and `class RandomStructureTransformation` to `standard_transformations.py`

[exenGT]

Closes #3049.

[mkhorton]

Thanks @exenGT, I added a question to the associated issue.

[exenGT]

@janosh All your suggestions have been addressed in my latest commits.

[codecov-commenter]

Codecov Report

Patch coverage: 86.36% and project coverage change: -0.57% :warning:

Comparison is base (5f369ec) 74.69% compared to head (b72f962) 74.12%. Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3057      +/-   ##
==========================================
- Coverage   74.69%   74.12%   -0.57%     
==========================================
  Files         230      230              
  Lines       69375    69419      +44     
  Branches    16154    16163       +9     
==========================================
- Hits        51818    51460     -358     
- Misses      14490    14923     +433     
+ Partials     3067     3036      -31     

Files Changed	Coverage Δ
...matgen/transformations/standard_transformations.py	86.04% <83.78%> (-0.22%)	:arrow_down:
pymatgen/core/structure.py	85.81% <100.00%> (+0.05%)	:arrow_up:

... and 7 files with indirect coverage changes

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