Report error if non-constant variable is specified as an input

[chrispcampbell]

See discussion #440 for more details. Currently the compiler will not throw an error in the case where an input variable specified in the spec.json file is not a constant, so if the user accidentally tries to configure a non-constant to be an input, the input values will not take effect and it is hard to diagnose. Only constants should be allowed to be configured as an input, so we should detect the case where an input is configured for a non-constant and report it as a hard error.

[ToddFincannonEI]

I would be happy to "fix this with documentation" saying "don't do that". It doesn't make much sense to set anything but a constant from inputs, so I doubt it comes up often in practice.