ScaledPrior and ShiftedPrior

[cscherrer]

This PR introduces two new prior wrapper types to the distribution module:

• ScaledPrior<Pr, Fx>: A wrapper for priors that scales the output distribution. If drawing a Pr gives a distribution Fx, then drawing ScaledPrior<Pr> will produce a Scaled<Fx>.

• ShiftedPrior<Pr, Fx>: A wrapper for priors that shifts the output distribution. If drawing a Pr gives a distribution Fx, then drawing ShiftedPrior<Pr> will produce a Shifted<Fx>.

[codecov]

Codecov Report

Attention: Patch coverage is 83.85269% with 57 lines in your changes missing coverage. Please review.

Project coverage is 80.89%. Comparing base (dbcf0f0) to head (aaca36a).
Report is 1 commits behind head on release/v0.19.0.

Files with missing lines	Patch %	Lines
src/dist/scaled_prior.rs	78.37%	32 Missing ⚠️
src/dist/shifted_prior.rs	82.26%	25 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##           release/v0.19.0      #35      +/-   ##
===================================================
+ Coverage            80.65%   80.89%   +0.23%     
===================================================
  Files                   98      100       +2     
  Lines                20080    20372     +292     
  Branches             20080    20372     +292     
===================================================
+ Hits                 16195    16479     +284     
- Misses                3770     3778       +8     
  Partials               115      115              

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[schmidmt]

Looks great!