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 we shouldn't have to worry about the numeric case here. I think that the logic of dealing with scalar elements of chebmatrices should be dealt with in the chebmatrix method.