Fix erdiagramgen issue where mermaid output contains enum variable name

[nicholsn]

This PR closes #1210 where the enum IdentifyingType.IDENTIFYING was being printed in the mermaid markdown output.

I added a __str__ method to the IdentifyingType enum so it'll render correctly.

Before

erDiagram
Dataset ||IdentifyingType.IDENTIFYING}o Distribution : "distribution"

Loading

After

erDiagram
Dataset ||--}o Distribution : "distribution"

Loading

[codecov-commenter]

Codecov Report

Merging #1433 (4aff2c4) into main (e38c90e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

@@           Coverage Diff           @@
##             main    #1433   +/-   ##
=======================================
  Coverage   80.23%   80.24%           
=======================================
  Files          78       78           
  Lines        9220     9222    +2     
  Branches     2244     2244           
=======================================
+ Hits         7398     7400    +2     
  Misses       1380     1380           
  Partials      442      442           

Impacted Files	Coverage Δ
linkml/generators/erdiagramgen.py	96.07% <100.00%> (+0.05%)	⬆️