param type deduction for UnionAll particle types

Now that many particle types actually have type parameters, I think this

InPartS.ParamType(::Type{MotileCell}) = MotileCellParams

should be

InPartS.ParamType(::Type{<:MotileCell}) = MotileCellParams

instead (and analogously for the other models). This problem goes unnoticed (for example in autogrid) if the cell type is only specified as MotileCell and the default type parameter is used, but not for explicitly specified type parameters.

Btw, why does ParamType have a default method returning AbstractParams? The error I got when I discovered this looked weird because of it. Would there be a downside to using @api_undef?