Support callable parameters in @mtkmodel

[cstjean]

As noted in #2995 (comment), callable parameters are a great new feature, that don't yet work inside of @mtkmodel.

@mtkmodel MyModel begin
	@parameters begin
		(interp::Tspline)(..)
	end
end

yields MethodError: Cannot convert an object of type Expr to an object of type Symbol while

@mtkmodel MyModel begin
	@variables begin
		x(t)
	end
	@parameters begin
		interp(::Float64)
	end
end

yields AssertionError: Multiple independent variables are used in the model Symbol Symbol (seemingly the ::Float64 is treated like the independent variable t)