insert_ions(): added temporary constant variables to improve code readability
main: adjust # of wl points to divide evenly into # of MPI procs
main: removed unnecessary dump of ray data at random wavelength index

Eventually we'll just print all ray data to HDF5.
misc: adde dIPython notebook file to read and parse MESA models
.gitignore: added IPython notebook checkpoint folder
AtomicLine: added missing distinction between Einstein B_ij and B_ji

They differ by a factor of g_i/g_j, so that affects all the eta's and
kappa's.