kramers kronig analysis tools and tests

Dear Everyone,

as a result from my work i have been using extensively the "kramer_kronig_transform" and "bethe_f_sum" methods. This has allowed me to understand and improve them greatly and now I think they may be ready for considering merging into master. So long, these two methods have been extended and bug-fixed, and also, four different nose-tests have been also added. These tests rely on the theoretical properties of inelastic scattering and surprisingly they work! :)

Both methods and tests are documented and commented throughout the code and I am willing to add hyperspy documentation if necessary. The nosetests in "test_kramers_kronig_transform.py" serve also as demonstration of the use of these tools.

If someone is interested please try them and comment issues.

FInally, big thanks to everyone for developing this great software together. I wish I could contribute more often!
Alberto Eljarrat.