Add initializer and initargs

[gwerbin]

Here is an initial draft implementation of #231.

Still to-do:

• New documentation
• Test cases
• Type hints

I also see a lot of missing argument and return type hints across the codebase. I can add some of those while I am working on this PR, if that's OK.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.72%. Comparing base (b78c63c) to head (e745151).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #232      +/-   ##
==========================================
- Coverage   91.37%   86.72%   -4.66%     
==========================================
  Files          12       12              
  Lines         580      580              
==========================================
- Hits          530      503      -27     
- Misses         50       77      +27     

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