Detector: print hits along with multiplicity of hit.

Also: print on detector positions and hits on seperate lines, added more
explanation to layer hit.
Made track a proper abstract class

Added the unimplemented decorator, and stopped calling the unimplemented init
function.
Added two way Kalman Filter.
Fixed: TwoWayLKFilter returned None on turning point.

Fixed that.
Slight changes to testing functions.
Added track generation function
Tracking test using new track generator.

Works wonderfully.
Stopped printing detector positions.

There are a lot of them and they just spam the terminal.
Fixed layer placements
Merge branch 'tracking' into detector
Finally wrote a proper readme.
Refectoring: made state a get-able and set-able property of the filter
Changed: filter will be able to step even if measurement is missing.

It will do this by skipping the update step and only performing the prediction.
Added: TwoWayLKFilter reverse function and refactored code to use it.
Added: get_layers inside LayeredDetector as a generator object.
Fixed bug in filter: undefined attributes raise AttributeError not NameError.
Added the fitter module.
Added fitting test.
Added: filter keeps track of the amount of iterations and optionally of
measurements.
Fitter now forces filters to keep track of measurements.