Constrained optimization for LM

[ConnorStoneAstro]

Is your feature request related to a problem? Please describe.
Need the capability to fit models where the parameters have constraints applied to them beyond equality constraints. Ideally with arbitrary flexibility.

Describe the solution you'd like
This can be done by adding virtual "pixels" to the image where the constraint is applied as a pixel value and some function of the parameters is the model.

Describe alternatives you've considered
so far equality constraints are available, but further constraints are needed to impose more general limits on the parameters.

Additional context
This can be thought of loosely as adding a prior. Alternatively it can be used to incorporate other non-imaging data like a rotation curve or SED into the fitting process.