Require that derivatives preserve controls

[goerz]

For non-linear amplitudes or generators, the derivative w.r.t a control should return an object whose controls are a subset of the controls in the original amplitude/generator. This must be an object identity, otherwise there will be hard-to-figure-out bugs. Previously, this requirement was not made explicit.

[goerz]

@stefan-o This should catch the issue we were discussing the other day

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.9%. Comparing base (fae356e) to head (e879915).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #87     +/-   ##
========================================
+ Coverage    85.7%   85.9%   +0.3%     
========================================
  Files          14      14             
  Lines         559     567      +8     
========================================
+ Hits          479     487      +8     
  Misses         80      80             

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.