Mark Parameter getters const

[eeberhard]

• To allow const class methods that get parameters or values, or to allow getting values on a const ParameterInterface, mark all getters as const and use const_pointer_cast on shared_from_this

[eeberhard]

Where do we need that?

I was going to justify this and tag reviewers later haha. I think getters should be marked const where applicable, both for clarity and for code safety. Currently any class that holds parameter properties isn't able to read the parameters in const methods. Sometimes you want or need those methods to be const to guarantee that nothing is changed.

For example, validating a parameter requires reading the value. If the validation fails, we don't want the parameter to be updated, so it should be const. If it's not const, a user-defined override might change it anyway which is not intentional.