Add __repr__ to function classes

[CnlPepper]

It is currently hard to debug errors in the function classes when they get heavily layered. It would be helpful to show the chain of the functions in the repr:

f = Exp1D(Constant1D(92.1) * Interpolate(x) / Constant1D(2.3)**Gain(x)))

displays as e.g.:

f = e^(92.1 * Interpolate(x) / 2.3**Gain(x))

It may also be useful for users to override the name of classes such as Interpolate, to make the data they are interpolating visible to the user:

f = e^(92.1 * LightIntensity(x) / 2.3**Gain(x))

[jacklovell]

+1 for this, though I would vote for __repr__ looking like your first code block, and having an additional __str__ for your second block. Then you could directly copy and paste the repr into a terminal to reproduce the object, which may also be useful for debugging.

This would mean the special arithmetic method functions (for __add__, __sub__ etc) would have identical __str__ and __repr__ representations, but more special cases such as the cmath functions, interpolators and Arg and Constant would need both methods.