Added the effect of photoionization and heating
by line photons generated elsewhere in the cloud. These are photons
which have already escaped the local region close to the point of emission.
Changed the step size computation algorithm in order
to account for the process of emission. That is, the step size is
now is based on the length scale for significant change of both absorption
and emission.
Fixed several errors in the atomic database, notably affecting
N-like ions. These affect some of the density sensitive lines in low ionization
models.
Update to this manual, in the chapter in the Physics of xstar,
describing in more detail the radiation transfer algorithm.