Fix bug: delegate parameter doesn't call its source to get value

[picarro-yren]

Bug description:
A DelegateParameter should redirect to its source for the value but instead <function Parameter.init.._get_manual_parameter> is called and 0.0 is returned.

Cause
The reason why DelegateParameter doesn't call its source for the value is because of the following lines in Parameter.init:

elif not self.gettable and get_cmd is not False:
      if get_cmd is None:
                self.get_raw = MethodType(_get_manual_parameter, self)

When external_parameter is created, its source is None so it's not gettable and its get_raw is thus bound to _get_manual_parameter.

Solution
It turns out that even though gettable property of DelegateParameter is False when source is None, _gettable attribute is True because get_raw is defined in parameter

So the solution we propose is to use _gettable attribute instead of gettable property in Parameter.init.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.37%. Comparing base (ba45309) to head (b677c47).
Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6671   +/-   ##
=======================================
  Coverage   69.37%   69.37%           
=======================================
  Files         340      340           
  Lines       31322    31328    +6     
=======================================
+ Hits        21729    21735    +6     
  Misses       9593     9593           

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