Resolve "Redesign Python interface"

Update python API

• New results class with read, write and get_spherical_average functions.
• New time-averager class to make @freeriks happy.
• arrays are now (N, 3)
• Resulting correlation function arrays should be (N_qpoints, time_window)
• spherical q-point averager return np.nan for q-bins without any q-points (instead of removing these from output)
• dsf renamed to dynasor (temporarily renamed cli to dynasor_cli)
• Removed C-backend
• Updated setup.py
• calculation now uses both F_AB and F_BA in calc_corr to get better statistics.
• Readers now returns ReaderFrame and Trajectory returns TrajectoryFrame which contains positions_by_type dict
• dynasor-cli now in bin and calls main() from dynasor/cli/main.py

Closes #60 Closes #78 Closes #54 (closed) Closes #50 (closed) Closes #102 (closed)