Gradient : derive deriver diff grad

### 2.50.1 Gradient : `derive deriver diff grad`

`derive` (or `diff` or `grad`) takes two arguments : an expression F of n real variables and a vector of these variable names.
`derive` returns the gradient of F, where the gradient is the vector of all partial derivatives, for exmple in dimension n=3

| ▸
grad
| (F)= [ |
∂ F
∂ x
| , |
∂ F
∂ y
| , |
∂ F
∂ z
| ]

Example
Find the gradient of F(x,y,z)=2x2y−xz3.
Input :

`derive(2*x```^```2*y-x*z```^```3,[x,y,z])`

Or :

`diff(2*x```^```2*y-x*z```^```3,[x,y,z])`

Or :

`grad(2*x```^```2*y-x*z```^```3,[x,y,z])`

Output :

`[2*2*x*y-z```^```3,2*x```^```2,-(x*3*z```^```2)]`

Output after simplification with `normal(ans())` :

`[4*x*y-z```^```3,2*x```^```2,-(3*x*z```^```2)]`

To find the critical points of F(x,y,z)=2x2y−xz3, input :

`solve(derive(2*x```^```2*y-x*z```^```3,[x,y,z]),[x,y,z])`

Output :

`[[0,y,0]]`