Enable dataset shape extent to be reduced

[jreadey]

Currently dataset shapes can only be made larger (up to max_size), not reduced. Support reduction of dimension extents similar to how the HDF5 library works.

This will require the ability for HSDS to delete chunks as they no longer are part of a dataset.

[mattjala]

Implemented in #266, though see #272 for a known issue with decreasing then increasing size.