Change the order of variables : reorder

### 2.24.24 Change the order of variables : `reorder`

`reorder` takes two arguments : an expression and a vector of variable names.
`reorder` expands the expression according to the order of variables given as second argument.
Input :

`reorder(x```^```2+2*x*a+a```^```2+z```^```2-x*z,[a,x,z])`

Output :

`a```^```2+2*a*x+x```^```2-x*z+z```^```2`

Warning :
The variables must be symbolic (if not, purge them before calling `reorder`)