Allow multiple parameters to dsl models

[richfitz]

Two somewhat related features:

• DSL models now support (crudely) handling multiple parameters (a matrix of parameters yielding a vector of densities)
• We can propagate the allow_multiple_parameters property when combining models

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.45%. Comparing base (c0a23ee) to head (b036c5f).
Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #99   +/-   ##
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  Coverage   99.45%   99.45%           
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  Files          69       69           
  Lines        5297     5362   +65     
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+ Hits         5268     5333   +65     
  Misses         29       29           

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