DiagonalCoeffs-->Band

* rename DiagonalCoeffs (clumsy name) to Band. I tried first Diagonal,
 but that name was already taken by a constant Diagonal which is used
 for various things already.
 * rename the template parameter DiagId to Index.
 * keep diagonal() unchanged, returns a Band<Derived, 0>
 * rename diagonal<int>() to band<int>()
 * introduce diagonal(int), returning a Band<Derived, Dynamic>. The
 trick here is as we already do in Block : use ei_int_if_dynamic to
 store the Index in Band.
   ---> that is the improvement that i need in order to code
 BandedMatrix. No need to unroll an outer loop there, and that also
 will reduce compilation times (only one Band type instantiated).