Fix neutron keys

Some of the keys in Sample were note properly skipped in neutron_cross_sections. This MR fixes that.

Also renames the following properties in the output Sample:

• S_q_w_coherent -> S_q_w_coh
• S_q_w_incoherent -> S_q_w_incoh
• S_q_w_total -> S_q_w

to be consistent with the input Sample.