Feature request: piecewise defined functions

[dqnykamp]

We need the ability to create a single function that is defined in a piecewise fashion.

For graphing, one can create multiple functions with suitably restricted domains, but constraining to such a combined function doesn't work as expected. The nearest point on each piece is calculated as a function, so it attempts to find a point on the same vertical line as the original point. However, the functions whose domain does not intersect that line yield an endpoint as the nearest point, which could be closer than the point on the given vertical line.