ENH : added option to use PIMS frame store for 2D

 - added new kwarg to `__init__ `use_pims_fs` which controls
   if 2D data is special cased to a different data structure than
   generic non-scalar data
 - allows for filename to be passed in as column data and handled in
   transparent way
 - updated tests
 - minor style changes to simplify code

closes #45
closes #42
closes #38