yumlgen: use POST and add ability to name output file

[jankatins]

We needed these two changes to
a) generate bigger diagrams
b) have more control over how the filenames end up

[codecov-commenter]

Codecov Report

Merging #1293 (c9ecc81) into main (4ceeba1) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head c9ecc81 differs from pull request most recent head d256e7e. Consider uploading reports for the commit d256e7e to get more accurate results

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@@            Coverage Diff             @@
##             main    #1293      +/-   ##
==========================================
+ Coverage   80.33%   80.35%   +0.02%     
==========================================
  Files          77       77              
  Lines        8995     9001       +6     
  Branches     2212     2212              
==========================================
+ Hits         7226     7233       +7     
+ Misses       1347     1346       -1     
  Partials      422      422              

Impacted Files	Coverage Δ
linkml/generators/yumlgen.py	94.73% <100.00%> (+1.58%)	⬆️
linkml/utils/schemaloader.py	78.92% <0.00%> (-0.19%)	⬇️

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[jankatins]

When I ran the tests, there were actually more changes to the currently checked in files but they were unrelated and I left them out.

Not sure why the tests do not complain about these discrepancies, either the tests are run twice or these changes do not register :-(

[cmungall]

@hsolbrig is working on refactoring the test framework...

[cmungall]

Many thanks! We are focusing efforts on mermaid but this still doesn't support everything supported by yumlgen, so this is most welcome

[jankatins]

Mermaid would definitely be nice, we also would want that: our goal in the end is to import the whole generated doc into a azure wiki (based on markdown files in a git, so simply comitting would work)