Add support for more math functions for Parameters and DescriptorNumbers

[henrikjacobsenfys]

This is to flesh out ideas for implementing #201

General

Right now, we support a few functions on parameters: add, sub, mul, div, pow, neg, abs. That's great, but we need a lot more: exp, trigonometric functions, sqrt, arguably also more complex ones like erf.

Idea

Make a class (MathFunctions?) to handle all the functions. It just needs to point at the scipp functions for most calculations, and handle propagation of bounds properly (see below). We can also point it at numpy functions to help us with speeding up calculations with dependent parameters by building in calculations that bypass the slow unit handling while still giving the right result (#298 ), and with implementing correct error propagation for dependent parameters (#299).

Bounds

Propagating the bounds for trigonometric functions can be handled in two ways.

1. the lazy one: we just set the bounds to the bounds of the functions (i.e. +-1 for sin)
2. the not so lazy one: we set the bounds based on the bounds of the parameter. So a sin function on a parameter with min and max that don't span the full range would have the correct bounds.