moved intro.
thermal intro placeholder.
rearranging transfer modes.
sectioned the sections
mass balance models.
adds paradigm and timesteps files
removes paradigm file
mostly deals with merge conflicts
moved interfaces to mass_transfer
moved figs in interfaces.
adds mass_transfer/intro
added intro file for nuclide_models.
removed errant section title.
added pphw notes.
changed a bunch of figure links... so tedious...