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: It's not clear from this what datatype we expect ```NEWTECH``` to be. Should it be a string, a function handle or an object of the tech type we want to convert to. Could you please clarify (and same for the chebmatrix method below).