change tests to accomodate for the change in arena coordinates
reorganise analysis modules

create a separate pkg: gridcells.analysis
add registration engine for rectangular arenas

unit tests seem to pass for very simple, artificially generated position
vectors
update documentation (all python code)

also remove breathe: it's so ugly the documentation of anything is pretty much
useless
polish registration engine interface

RegistrationEngine is more suitable for a base class. Changed to
ArenaOriginRegistration.
Merge branch 'position-registration'