U_matrix supports wannier90 basis

[harrisonlabollita]

The functions that transform the Umatrix: spherical_to_cubic, t2g_submatrix, and eg_submatrix now support convention = "wannier90", where the d-shell is ordered as ("z^2", "xz", "yz", "x^2-y^2", "xy").

[harrisonlabollita]

Conventions have been explicitly named for the construction of the U_matrix that correspond to popular DFT codes. The previous default convention '' (an empty string) has been renamed to triqs to avoid any ambiguity.

[the-hampel]

Okay now everything looks good. Maybe @Wentzell can have a quick look, thank you.

[Wentzell]

Thank you @harrisonlabollita for these improvements!