In Dataset Model, variables can be blank.  Cleanup of debug print statements.
Add axis parameter to numpy.append. Use filled() method in MaskedArray.
Improve NaN handling, check change of dataset in session, add var plot_type.

Detect if user has changed datasets.
In JSON encoder, convert NaNs to None for HighCharts.
Set plot_type on each variable based on its dimension.
Enable admin for Project, Platform, Dataset