Made test_read_standard_arrays more precise in test_netcdf_outputter. It
now raises an error if assertions are not tested due to no matching data
found -- this exercises changing output_timestep
Wrote better tests for testing write_output_post_run with variable
output_timestep
- Updated outputters so if write output falls in middle of step, the
  output is writtin for current step and keep track of fractional time
  remaining. It ensures the output is written for closest matching model step.
  Therefore, if output_timestep is output at 1.5 times model
  time, then output happens at steps: [0, 2, 3, 5, 6, ..]
- defined internal _output_timestep which is an int (in seconds), while
  the property exposed to user (output_timestep) is still timedelta
  since user defines this as timedelta(days=1) if output is desired
  every 24hrs after model start time
- updated base class prepare_for_model_run to take and set
  model_start_time and sc_pair attributes since multiple outputters
  require these
Tweaked test_output_timestep a bit more
GeoJson outputter should send a FeatureCollection directly instead of a file reference.
Merge branch 'master' of https://trac.orr.noaa.gov/git/PyGnome
Merge branch 'master' of https://trac.orr.noaa.gov/git/PyGnome
updated test to reflect the change in commit 97023a28