Added a load method to the Dat class.
Ensure that 'filename' has a .npy extension.
Handle the Dat and MixedDat cases separately.
Add a test for the 'load' and 'save' Dat methods.
Put the output file in a temporary directory.

Applies Lawrence's diff in pull request #385.
Merge pull request #385 from ctjacobs/dat-load-from-file

* dat-load-from-file:
  Put the output file in a temporary directory.
  Add a test for the 'load' and 'save' Dat methods.
  Handle the Dat and MixedDat cases separately.
  Ensure that 'filename' has a .npy extension.
  Added a load method to the Dat class.