Description

The compiled constraints are supposed to be a function that takes the parameters as input, and updates the values of the parameters based on the constraint expressions supplied, after each call to problem.setp(pvec), through the setp_hook() function.

The existing code (introduced in #3375) was incorrectly replacing the parameter names with f"{name}.slot", which is wrong for 2 reasons:

1. In the case of simple equality constraints, e.g. P1 = P2, it was instead executing P1.slot = P2.slot, which replaces the Variable in the slot of P1 with the actual Variable instance in P2, so that the P1 is essentially an alias for P2 after that. This will "work" and the fit will complete, but the parameter structure of the model is changed in an unintended way.
2. In the case of (even slightly) more complex constraints, the constraint expressions fail to execute properly, e.g. for this constraint expression:
   M1.scale = M2.scale * 1.2 you end up with this expression to evaluate: M1.scale.slot = M2.scale.slot * 1.2, which doesn't work because M2.scale.slot is an instance of Variable, on which we can't directly do arithmetic.

To achieve the goal of the compiled constraint functions, we should instead use the .value attribute; on the LHS of the equality, it will trigger the .value setter which pushes the new value down into the slot, and on the RHS it will use the getter to pull the value back out of the slot for (as a float) for doing expression math on it.

This fixes a sub-issue in #3793 (comment)

How Has This Been Tested?

Ran the constrained fit in this project:
6-1-2-constrained-fit-expr.json

Review Checklist:

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Documentation (check at least one)

• There is nothing that needs documenting (fixes a bug)
• Documentation changes are in this PR
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Installers

• There is a chance this will affect the installers, if so
  • Windows installer (GH artifact) has been tested (installed and worked)
  • MacOSX installer (GH artifact) has been tested (installed and worked)
  • Wheels installer (GH artifact) has been tested (installed and worked)

Licensing (untick if necessary)

• The introduced changes comply with SasView license (BSD 3-Clause)