coerce SCE to/from RSE/SE

[jkanche]

Implements: #71

[codecov]

Codecov Report

❌ Patch coverage is 82.35294% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.50%. Comparing base (5116d37) to head (9a9e541).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/singlecellexperiment/SingleCellExperiment.py	82.35%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
+ Coverage   68.12%   68.50%   +0.38%     
==========================================
  Files           7        7              
  Lines         618      635      +17     
  Branches       86       86              
==========================================
+ Hits          421      435      +14     
- Misses        155      158       +3     
  Partials       42       42              

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