Expand and revamp model and symbol constraints

[clegaspi]

As it is put in some of the code comments, constraints are a bit "hacked together" at the moment. :)

Constraints should probably be set apart as their own class, instead of being a subclass of Model. I can see it was done initially for the sympy parsing and plug in, but models have much more functionality and metadata that constraints don't need or need differently. Maybe there just needs to be more overridden functions. I haven't taken a very close look at it, so it's hard to say right now what is the best course of action.

Also, current implementation does not really support constraints that involve non-numeric quantities/symbols. For example, if a model only applies to non-magnetic materials, we should have a constraint magnetic_order == "NM". At present, sympy can kind of figure this out, but not really. A thought would be to generate lambda functions for these kinds of expressions or to use eval() and warn users to not import any YAML files from untrusted sources.

[maxhutch]

Do you have any concrete examples of constraints that aren't independent (i.e. rectangular)? For example, not isMetal and resistivity > 1.0e-3 and refractive_index > 1.5 would be independent but resistivity * refractive_index > 1.5e-3 would not be.

[clegaspi]

Currently, no. Although the current implementation should work for dependent (non-independent?) constraints.