Zeroth order version of SampleSet

Untested, probably full of problems. But I hope this is enough that JHP can
add storage support.
Finished draft of SampleSet.

Now I need to start writing tests for it to make sure it does what I think
it does.
Skeleton of tests for SampleSet.
Fixes based on first group of tests.
Fixes to SampleSet.__delitem__ and more tests.
SampleSet: more tests, better docs.
SampleSet: added SampleKeyError. And more tests.
Merge branch 'sample' into global_movers

Conflicts:
	opentis/trajectory.py