Facilitate extraction from IOF coarsened SalishSeaCast model products

[douglatornell]

Greig & Michael's coarsened grid SalishSeaCast model products are stored as day-average fields in month long files; e.g.

  SalishSea1500-RUN203_1d_grid_T_2D_y1984m04.nc
  SalishSea1500-RUN203_1h_grid_U_y1984m04.nc
  SalishSea-RUN203_1d_grid_U_y1984m04.nc

The file patterns are like:

file pattern: "{yyyy}/SalishSea1500-RUN203_1d_grid_T_2D_{nemo_yyyymm}.nc"

PR #40 provided the yyyy() and nemo_yyyymm() date formatter functions to handle this.

So, core.extract.calc_ds_paths() needs to have a way of stepping through the date range 1 month at a time.

This is somewhat related to issue #41.