Truncate of order n : truncate

### 2.24.21 Truncate of order n : `truncate`

`truncate` takes as argument, a polynomial and an integer `n`.
`truncate` truncates this polynomial at order `n` (removing all terms of order greater or equal to `n+1`).
`truncate` may be used to transform a series expansion into a polynomial or to compute series expansion step by step.
Input :

`truncate((1+x+x```^```2/2)```^```3,4)`

Output :

`(9*x```^```4+16*x```^```3+18*x```^```2+12*x+4)/4`

Input :

`truncate(series(sin(x)),4)`

Output :

`(-x```^```3-(-6)*x)/6`

Note that the returned polynomial is normalized.