Add template api

[bollwyvl]

• hoist template stuff to IDiagramManager
• rename Advanced Diagram to Custom Diagram
• tweak conda test for windows
• more docstrings

[codecov]

Codecov Report

Merging #42 (0a2f211) into master (e2b33bc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #42   +/-   ##
=======================================
  Coverage   96.52%   96.52%           
=======================================
  Files          21       21           
  Lines         576      576           
  Branches       33       33           
=======================================
  Hits          556      556           
  Misses         12       12           
  Partials        8        8           

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