Linearize exponentials : lin

### 2.23.4 Linearize exponentials : `lin`

`lin` takes as argument an expression with exponentials.
`lin` rewrites hyperbolic functions as exponentials if required, then linearizes this expression (i.e. replace product of exponentials by exponential of sums).
Examples

- Input :
  `lin(sinh(x)```^```2)`
  Output :
  `1/4*exp(2*x)+1/-2+1/4*exp(-(2*x))`
- Input :
  `lin((exp(x)+1)```^```3)`
  Output :
  `exp(3*x)+3*exp(2*x)+3*exp(x)+1`