CLIMATE-396 - Ensure RCMED parameters are cast properly

- Cast RCMED dataset id and parameter id to integers before attempting
  to load a dataset.
Added more unit tests for normalize_lat_lon_values() helper
CLIMATE-397 - Switch UI backend over to safe_subset

- The UI backend now uses dataset_processor.safe_subset instead of
  subset when it performs subsetting tasks on the loaded datasets. This
  helps to ensure that RCMED datasets don't wind up causing problems due
  to fractional lat/lon grids.
Resolve CLIMATE-395. Merge PR #13.
Resolve CLIMATE-396. Merge PR #14.
Resolve CLIMATE-397. Merge PR #15.