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 remote-tracking branch 'dwhswenson/sample' into samplestorage
moving classes
use netCDF vlen for trajectories
use netCDF vlen for trajectories
fix error in base_cls
fix error in base_cls
cleanup of Storage
fix orderparameter to use Configurations not Snapshots
added storage support for SampleSet
fixing
Added replica_list and ensemble_list methods

These return all the replica IDs and all the ensemble names, respectively,
by just returning the keys to the appropriate dictionaries.
simplify caching
simplify caching