CLIMATE-390 - Fix broken map layers

- The URL for the map tile layer that the Leaflet and Preview map
  directives were using is no longer valid. It has been replaced with a
  new one from the LeafletJS website.
CLIMATE-391 - Normalize evaluation time values in OCW UI

- Date values that are passed for an evaluation are now normalized so
  all the day values are 1 unless the evaluation is handling daily data.
CLIMATE-392 - Shift lat/lon grid during Dataset creation

- When creating a Dataset object with a lon grid that ranges from 0-360
  the values are now shifted to fall within [-180, 180).
CLIMATE-393 - Add dataset_processor.safe_subset

- Add safe_subset for gracefully handling subsetting when not all of the
  bounding parameters are fully contained in the target dataset. If any
  of the bounding values fall outside of the dataset's bounds they are
  defaulted to the dataset's maximum/minimum.
CLIMATE-392 - Add lat/lon normalization helper

- Add Alex Goodman's helper for normalizing lat/lon grids and ensuring
  that they're monotonically increasing. This is significantly more
  robust than the current shift that is being done on dataset load.
CLIMATE-392 - Minor testing for new helper

- Add a really simple sanity check test for the new
  normalize_lat_lon_values helper. We definitely need to add more tests
  for this when time permits.
CLIMATE-392 - Use new utils helper for lat/lon clean up

- Use the utils.normalize_lat_lon_values helper for cleaning up any
  lat/lon problems when creating a new Dataset object.
CLIMATE-380 - Fix bad ref_dataset reference
CLIMATE-394 - Temporarily hide subregion input box
CLIMATE-392 - Documentation clarifications
Resolve CLIMATE-380. Merge PR #11.
Resolve CLIMATE-390. Merge PR #7.
Resolve CLIMATE-391. Merge PR #8.
Resolve CLIMATE-392. Merge PR #9.
Resolve CLIMATE-393. Merge PR #10.
Resole CLIMATE-394. Merge PR #12.