set the photoionization cross-sections

Change the photoelectric absorption cross-sections in use.

Syntax: xsect [bcmc|obcm|vern]

The three options are: bcmc, from Balucinska-Church & McCammon (1992; Ap.J.400, 699) with a new He cross-section based on (1998; Ap.J. 496, 1044); obcm, as bcmc but with the old He cross-section, and, vern, from Verneret. al. (1996 Ap.J.). This changes the cross-sections in use for all absorption models with the exception of wabs and tbabs.