3D parametric surface : plotparam paramplot DrawParm

### 3.11.2 3D parametric surface : `plotparam paramplot DrawParm`

`plotparam` takes two main arguments, a list of three expressions of two variables and the list of these variable names where each variable name may be replaced by variable=interval to specify the range of the parameters. It accepts an optionnal argument to specify the discretisation steps of the parameters u and v with `ustep=...` and `vstep=...`.
`plotparam([f(u,v),g(u,v),h(u,v)],[u,v])` draws the surface defined by the first argument : x=f(u,v),y=g(u,v),z=h(u,v), where u and v ranges default to the graphic configuration.
Input :

`plotparam([v*cos(u),v*sin(u),v],[u,v])`

Output :

`The cone` `x=v*``cos``(u),y=v*``sin``(u),z=v`

To specify the range of each parameters, replace each variable by an equation variable=range, like this:

`plotparam([v*cos(u),v*sin(u),v],[u=0..pi,v=0..3])`

Output :

`A portion of the cone` `x=v*``cos``(u),y=v*``sin``(u),z=v`

Input :

`plotparam([v*cos(u),v*sin(u),v],[u=0..pi,v=0..3],ustep=0.5,vstep=0.5)`

Output :

`A portion of the cone` `x=v*``cos``(u),y=v*``sin``(u),z=v`