Added: Detector can now propagate multiple tracks.
Changed: test modules make use of propagate_tracks

Makes it slightly easier to propagate multiple tracks.
Added: x_step property to LayeredDetector.

And change in get_layers docstring.
Fixed: 2WLKF finally has a constructor.

This enables clean-up in reverse function.
Changed: 2WLKF variable detect now has the full name.
Indentation fixes
Changed: reversed_measurements are [] by default.

Allows the usage of len.
Changed: fitter discards filters with < 3 measurements.
Added: fitter propagation function.
Add one iteration before and after.
Detector prints hits with rounding now.
Added measurement list property to filter (returns measurements as list instead of list of matrices).
Variable name changes to fitter.
Fixed fitter:

Since measurements are first updated and then predicted, the fitter was
returning the measurements moved by one. So I moved the extra final filter step
to before the loop over the measurements. That way the filter is at the first
measurement when the loop begins and there is no shift between prediction and
measurement.
Modified test_fitting to remain up-to-date with all the changes.
Fixed bug in filter:

Measurements can be None, which means they can't be subscripted, added a check.
Detector: x_step should always be float (just in case).
Changes to test_fitting: played with parameters.
Bugfix: y_err must be different than zero - must be calculated as float.

y_err is used in the R matrix (measurement covariance). Since numerical
precision is limited, it is necessary that this value be non zero.

Also slight tuning of parameters.
Added TODO in fitter.