Example that computes integrals with a NN

[francesco086]

Build an example for MCI++:

compute the integral

     \int_{-10}^{+10} dx x^2 nn(x)^2

(nn(x) is a normalized neural network) with MC and a non-MC method

do the same with

     \int_{-10}^{+10} dx \laplacian^2 nn(x)^2

Add this example in the MCI++ library repository.

See ex2

[francesco086]

I replaced the term lapl2 with nabla2 throughout the code, and mentioned at the beginning of the main.cpp that nabla2 is the laplacian operator

[francesco086]

I fixed the part of the code that was computing the kinetic energy. I was computing \int (\nabla^2 nn) nn^2 instead of \int (\nabla^2 log(nn)) nn^2

[Ithanil]

Fine. Merge.