Better descriptions for random initializations

[mstimberg]

With the polishing branch, intializations that refer to rand() or randn() already look a bit nicer. But I think we could use this as a first example of automatic annotation with additional analysis.
If an expression only contains a single rand() call, we can use sympy's substitution mechanism and substitute it once with 0 and once with 1, and then use this to display the lower and upper bounds. So if you have an initialization like:

v = El + (V_th - El)*rand()

This would currently be displayed as:

but we could further annotate this with something like

We could probably do something fancy with sympy as well to get the mean and variance of a normal distribution if the expression uses randn().