Feature: evaluate functions on screen at different x-value

[mariano-f-r]

As @SilverScan has pointed out to me (and to himself), while it's nice to be able to graph functions (and their derivatives), and roughly see where everything is, it's also useful to evaluate these functions at arbitrary x-values. This should be relatively simple, as functions are stored internally after they are graphed. This would be as simple as just evaluating said functions. A similar list could be created for derivatives. Implementation is not important :P

[SilverScan]

Appreciated, will get to work on this some point in the near future