Exponential map of DifferentialOperator: diff_op_exp

A simple function ```diff_op_exp()``` to perform the exponential map of a DifferentialOperator. It depends on ```pdsolve```, it failed in most cases, but works for a simple differential operator (thus generating the translation operator).

It provides a trick to settle the initial condition ```f(0, x) == f(x)```.

In order to have better support it is necessary to improve the ```pdsolve``` function.