#615 Derivative should really throw errors when their input is not differentiable

[High Performance Coder]

As a comprormise, we might soften the condition to continuous and mostly differentiable. If f(x) is continuous andm mostly differentiable, then \int f'(x)dx = f(x). A case in point is the absolute value function. However functions like < which are not continuous at 0, will not satisfy \int f'(x)=f(x).

[High Performance Coder]

So derivatives of discontinuous functions will now throw an exception.