Allow Functional Bounds on Infinite Variables

[pulsipher]

Currently, we allow start values on infinite variables to be functions. However, bounds can only be constants as illustrated in #252.

Hence, we should generalize the API to allow for functional bounds. This will likely have implications in relation to #148.

This new API would look something like:

using InfiniteOpt

model = InfiniteModel()
@infinite_parameter(model, t in [0, 1])

my_func(t) = sin(t)
@variable(model, -my_func <= y <= myfunc, Infinite(t))