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 I'm not a big fan of the name of this method (and the corresponding chebmatrix method). It implies that before calling it, a chebfun could potentially have an incorrect tech, which is only relevant for the chebop class. Couldn't we just call this method ```changeTech```?