Cleaning up CHEBOP class.

(1) Add a convertToCorrectTech method.
When we solve an ODE with Fourier collocation, we have to convert the rhs and the initial guess to trigfuns, if they are not trigfuns yet. Before, we did the conversion without testing it they were trigfuns yet and for any tech. Now, we do the conversion only if necessary.

(2) Use a sparse representation for the Projection Matrix P when P is the identity.

(3) Coding style adjustment. 


@Hadrien-Montanelli two questions:

* Why is the method called in the static method syntax?
* Shouldn't this be calling ```changeTech``` rather than ```convertToCorrectTech```?