Make PSDMatrices <: AbstractPDMat to use them with Distributions.jl

[nathanaelbosch]

Left to do: Check that ProbNumDiffEq.jl is compatible with this

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (e380ae7) 98.21% compared to head (5bbaf81) 96.90%.

❗ Current head 5bbaf81 differs from pull request most recent head 0d211d5. Consider uploading reports for the commit 0d211d5 to get more accurate results

Files	Patch %	Lines
src/PSDMatrices.jl	95.45%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
- Coverage   98.21%   96.90%   -1.31%     
==========================================
  Files           1        1              
  Lines          56       97      +41     
==========================================
+ Hits           55       94      +39     
- Misses          1        3       +2     

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