Mwahpy

- fixed sky_to_pole() tests and a couple other tests
- fixed a bug with output_handler.make_nbody_input() not writing '\n' at end of file

- sky_to_pole() now uses a change of basis matrix instead of vector math
- fixed bugs with mismatches of Timestep.index_list after extra calculations
- Timestep.id is now an array of ints, not floats
- added Timestep.take_by_id()
- output_handler.make_nbody_input() now properly handles the 'recenter' keyword argument
- output_handler.make_nbody_input() pads now floats correctly
- updated README
- Timestep.typ is now a list of ints, not floats

- Timestep.typ attribute stores whether particles are baryons (0) or dark matter (1)
- output_handler has been updated to also read in particle type
- Timestep.only_baryons() and Timestep_only_dark_matter() cut the data to only
include one type of particle
- Added support for galactic spherical coordinates (phi, theta)
- Added find_progenitor()
- make_nbody_input() no longer recenters the Timestep before printing out
- fixed 'self not defined' error in output_handler.make_csv()
- Timestep.rad and Timestep.rot are now Timestep.vrad and Timestep.vrot
- updated documentation pdf to new naming convention
- updated documentation to include all supported functions
- updated README
- removed coords.approx_lsr()

- added changelog
- fixed bug in Timestep.copy() that would cause error if the Timestep had calculated values
- adapted python standard naming convention (mostly, method names went from camelCase to lower_case)
- consolidated coords.py tests into tests.py
- bug fix in coords.cart_to_plane()
- fixed import scheme