Adding survey S/N degradation

Following Bagchi & lorimer, added fortran functions to calculate
degradation of S/N for linear, acc, and jerk searches for pulsars in
binaries. Need to add orbits to pulsar class (will use orbit.py base
class).
Adding orbit functionality
Functionality all added

Need to test vs the results from the raw fortran code.
orbit.py

Removed comments which aren't necessary!