Relax conditions on data when writing to observables
- allow scalar observables
- don't insist on matching dtypes (h5py will do the conversion)
- raise an exception if the shape of a value doesn't match,
  rather than silently dropping the value
Make center_of_mass a scalar observable in random walk example
Merge pull request #3 from khinsen/better-observable-writing

Relax conditions on data when writing to observables