Class that represents a diskdf as initial DF + subsequent secular evolution
NAME:
__init__
PURPOSE:
initialize
INPUT:
initdf - the df at the start of the evolution (at to)
pot - potential to integrate orbits in
to= initial time (time at which initdf is evaluated; orbits are integrated from current t back to to)
OUTPUT:
instance
HISTORY:
2011-03-30 - Written - Bovy (NYU)