migrate FEFF input generation by using `larixite.struct` module

[maurov]

Motivation

There are three implementations of the FEFF input generation:

• Struc2XAS.make_input_feff() in the module larixite.struc2xas, which is a mere copy of larch.xrd.struct2xas;
• cif2feffinp in the module larixite.cif_cluster (currently used by the Larixite web application and the Larix Wx GUI);
• structure2feffinp in larch.xrd.structure2feff

Solution

Merge in a single code following the approach of the FDMNES input generation in larixite.struct.