Warn for models where input parameters are redefined in the model

[mattfidler]

          I think rxode2 should at least throw an warning if not an error if an endpoint or a secondary parameter is found in the list of covariates for a model.

Originally posted by @john-harrold in #829 (comment)

I think that a broader warning that is closer aligned to what rxode2 parses is to warn on parameters that are both input parameters and variables assigned in the model.

An example that sometimes occurs are things like:

wt <- wt/70

Some models will not work without this enabled, so if anything it should be a warning.