Linearize exponentials : lin

### 5.26.4 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