Fix Billiard Motion parameters

[Kyle Williams]

Currently the billiard motion init function takes in the total number of sensors as a parameter. This should be changed to the number of interior sensors since it is redundant and possibly confusing. If we need the total number of sensors, we can always use len(boundary) + n_int_sensors. Additionally, it requires the user to know how to compute the total number of sensors, and remember to use it as an input.