Avoid overloading dimensions on paramfile

_Originally posted by @mvdebolskiy in #3414; original post deleted to keep that issue focused.

> Here are mimics parameters that reuse existing segment and variants dimensions/coordinates.
> Base, exponent (variants):
> `mimics_desorp, mimics_fchem_k, mimics_fchem_r, mimics_fphys_k, mimics_fphys_r,mimics_p_scalar, mimics_tau_r`
> Misc (variants):
> `mimics_fi` (fractions for SOMp SOMc)
> Misc (segments):
> `mimics_fmet` (this one is 4 parameters in 1 eq)
> Should there be an unbounded dimension on the paramfile that is used for these types of parameters? Or there should be more explicitly named dimensions? Right now there are some explanations in the variable attributes.

These should definitely just have their own named dimensions.