Transform trigonometric functions into complex exponentials : trig2exp

### 2.21.19 Transform trigonometric functions into complex exponentials : `trig2exp`

`trig2exp` takes as argument an expression containing trigonometric functions.
`trig2exp` rewrites the trigonometric functions with complex exponentials (without linearization).
Input :

`trig2exp(tan(x))`

Output :

`((exp((i)*x))```^```2-1)/((i)*((exp((i)*x))```^```2+1))`

Input :

`trig2exp(sin(x))`

Output :

`(exp((i)*x)-1/(exp((i)*x)))/(2*i)`