3D implicit surface

### 3.10.2 3D implicit surface

- `plotimplicit(f(x,y,z),x,y,z)` draws the graphic representation of the surface defined by the implicit equation f(x,y,z)=0,
- `plotimplicit(f(x,y,z),x=0..1,y=-1..1,z=-1..1)` draws the surface defined by the implicit equation f(x,y,z)=0, where 0≤ x ≤ 1, −1≤ y ≤ 1 and −1≤ z ≤ 1.

It is possible to add three arguments to specify the discretisation steps used for x, y and z with `xstep=...`, `ystep=...` and `zstep=...`.
Input :

`plotimplicit(x```^```2+y```^```2+z```^```2-1,x,y,z, xstep=0.2,ystep=0.1,zstep=0.3)`

Input :

`plotimplicit(x```^```2+y```^```2+z```^```2-1,x,y,z, xstep=0.2,ystep=0.1,zstep=0.3,unfactored)`

Output :

`The unit sphere`

Input :

`plotimplicit(x```^```2+y```^```2+z```^```2-1,x=-1..1,y=-1..1,z=-1..1)`

Output :

`The unit sphere`