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.