Fix semisimple -> simple in symmetry reduction

[blegat]

First step of #302

Closes #307
Closes #306
Closes #305
Closes #304
Closes #303

[codecov]

Codecov Report

Patch coverage: 88.50% and project coverage change: +3.77 🎉

Comparison is base (8476c96) 80.11% compared to head (15066a1) 83.89%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #308      +/-   ##
==========================================
+ Coverage   80.11%   83.89%   +3.77%     
==========================================
  Files          41       41              
  Lines        2112     2179      +67     
==========================================
+ Hits         1692     1828     +136     
+ Misses        420      351      -69     

Impacted Files	Coverage Δ
src/Certificate/Symmetry/block_diag.jl	90.72% <88.50%> (+90.72%)	⬆️

... and 1 file with indirect coverage changes

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