Added pivoting to matrix inversion, making it work for all invertible matrices.

Modified the test procedure test_matrix to work with the new argument list.
Fixed: measurements is now empty list if empty, not none.

Thanks to this the builtin pop can safely be used and less checking is needed.
Added detector module.

The detector module implements a detector interface and a layered detector
consisting of layers of strips. The detector can be drawn.
Fixed: Need to compare to None with is not ==.
Added track module.
Fixed: strip handling in layer.

Spawns strips with proper y position, only add strip to hit_strips if it isn't
already there.
Added hit clearing to detectors.
Added track module.
Added drawing of hits.
Fixed 2 bugs in Layer.hit: num_strip needs to be an integer and case of hitting the edge of the top-most strip.
Added propagate track to detector.
Draw example tracks along with detector.
Made test for tracking, moved from detector module.
Merge branch 'detector' into docs

Conflicts:
	matrix.py
Resolved!
Added docs for detector and track modules.
Added links to docstrings in detector.
Indexing into classes.