Version 0.8.1

Changes:

• Add support for Gaussian PSF (see #2).
• Rename function hash_ to hashfunc to remove conflicts with variables names hash_ in the code.
  For user code, add a compatibility function hash_ that raises a FutureWarning.
• Fix previously ignored chunkslice argument. This argument is either 'time' or 'bytes' and determines
  if chunksize refers to the number of elements or bytes.
• Code and docstrings cleanups.

This release was sponsored by Dr. Eitan Lerner of the Department of Biological Chemistry, in the Alexander Silberman Institute of Life Sciences, The Hebrew University of Jerusalem.