Allow user to choose specific time and variables.

Make mongoimporter read available time range in netcdf file and
put it into database. Add GUI crontrol that shows and allows
user to choose a particular time step from the range to load.
Make the time steps selectable in the GUI.
Feed user's choice down to vtk reader.

Similarly hookup the path for choosing a particular array.
Merge remote-tracking branch 'origin/select_timestep_and_variables' into next

* origin/select_timestep_and_variables:
  Allow user to choose specific time and variables.

Conflicts:
	modules/vtk_reader.py