Fix method ambiguity of `*` with `PDiagMat`

[devmotion]

Fixes #176.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.64%. Comparing base (53bf0d0) to head (b2dffb0). Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #205      +/-   ##
==========================================
+ Coverage   91.61%   91.64%   +0.02%     
==========================================
  Files           9        9              
  Lines         680      682       +2     
==========================================
+ Hits          623      625       +2     
  Misses         57       57              

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