Change the photoelectric absorption cross-sections in use.
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.