generalize U_matrix_kanamori and fix U_matrix doc

[the-hampel]

• generalize U_matrix_kanamori to allow for Uprime != U-2J cases
• allow to generate full four index Uijkl tensor as in U_matrix_slater
• rename U_matrix to U_matrix_slater because a function should not have the same name a module (sphinx autodoc currently broken see emptiness of https://triqs.github.io/triqs/latest/documentation/python_api/triqs.operators.util.U_matrix.html )
• clean up weird before function comments and merge missing info into docstrings

Warning: this will break backward compatibility due to renaming U_matrix -> U_matrix_slater !!! Please discuss.

[Wentzell]

@krivenko Could you have a look at this?

[the-hampel]

Hi @Wentzell @krivenko , thank you for your suggestions. I fixed the flawed if statements, extended the docstrings, and added a unit test as suggested.