Linearize a trigonometric expression : tlin

### 2.21.3 Linearize a trigonometric expression : `tlin`

`tlin` takes as argument an expression containing trigonometric functions.
`tlin` linearize products and integer powers of the trigonometric functions (e.g. in terms of sin(n.x) and cos(n.x))
Examples

- Linearize cos(x)*cos(y).
  Input :
  `tlin(cos(x)*cos(y))`
  Output :
  `1/2*cos(x-y)+1/2*cos(x+y)`
- Linearize cos(x)3.
  Input :
  `tlin(cos(x)```^```3)`
  Output :
  `3/4*cos(x)+1/4*cos(3*x)`
- Linearize 4cos(x)2−2.
  Input :
  `tlin(4*cos(x)```^```2-2)`
  Output :
  `2*cos(2*x)`