Issue 40

Fixes #40 

(Note: This depends on #48, which should be merged first.)

With MathFunction.solve(), it was pretty trivial to add not only linear x- and y-intercepts, but (potentially) multiple x-intercepts for all functions that we know how to solve.

If we really want to restrict this to just linear functions, that's pretty trivial too: we would just need to wrap the call to printIntercepts() with something like:

    if (mf.isLinearFunction() && !mf.isConstant())