Add second cross derivative

[francesco086]

In the future it might be useful to have second order cross derivatives for the FFNN (i.e. d/dbeta d^2/dx^2), for:

• fitting, requiring also the second derivative to look similar to the original function
• for VMC optimization, it might be required to compute H d/dbeta Psi (http://lanl.arxiv.org/abs/cond-mat/0611094v2)

AC:

• the FFNN library should have a method for calling this derivative
• the derivatives are tested in a unittest
• update the documentation consistently