DelegateParameters inherit their source's settable/gettable/snapshot_value/unit/label

[samantha-ho]

This PR fixes a bug observed when chaining multiple DelegateParameters, and changing the source at the bottom of the chain.

In the previous implementation, a DelegateParameter only updated it's internal _settable and _gettable attributes when the source was changed. But it is possible to set up a chain of DelegateParameters and then change the 'root' source Parameter. This updates the bottom-most DelegateParameter, but all of the other DelegateParameters in the chain will not update.

This PR changes the implementation to override the ParameterBase implementation of the gettable and settable properties, so that they reflect the underlying source parameter, no matter what it may be.

[samantha-ho]

@jenshnielsen
One possible issue... there is a seemingly rarely used _SetParamContext manager in the ParameterBase module that directly sets the parameter._settable attribute. This implementation will not properly handle that case, as the internal _settable and _gettable attributes on a DelegateParameter do not get used.

Not sure if this is a big enough issue to rethink the implementation. What are your thoughts?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.37%. Comparing base (44cd451) to head (9391665).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6648      +/-   ##
==========================================
+ Coverage   69.35%   69.37%   +0.01%     
==========================================
  Files         340      340              
  Lines       31303    31322      +19     
==========================================
+ Hits        21710    21729      +19     
  Misses       9593     9593              

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