3d function plots

3d function plots

We introduce analogues of graph.data.function etc. to be used as data sources for graph.graphxy.
Naming is changed in order to make things more consistent with grah/graphxy.

    * rename functionxy to functionlambda (2D function given as lambda expression)
    * rename paramfunctionxy to paramfunctionlambda (2D parametric function given as lambda expression)
    * new functionxy (3D function given as textual expression)
    * new functionxylambda (3D function given as lambda expression)
    * new paramtsfunction (function of 2 parameters given as textual expression)
    * new paramtsfunctionlambda (function of 2 parameters given as lambda expression)

Note that all parametric functions can provide 2D as well 3D data.