Implementation of graded layers

[lnacquaroli]

Consider adding a keyword accepting the type of layer depending on the index variation: constant (default) as usual, graded exponential (see issue 29), and maybe others.

Depending on the type of layer the matrix would be the usual one with the constant layers, and in the case of the exponential graded would be the matrix with Bessel functions (see paper in issue 29).

There might be other types with different graded as well that We could search and include if they exist.