MPAS native output support + example

[zmoon]

• unstructured grid support in contours and identify, assumed if only one spatial dimension
  • Using Delaunay triangulation on lat/lon grid points (or user-provided Triangulation) + Matplotlib tricontour
• example with subset of native MPAS output, same time and location as the existing (regridded) MPAS example dataset
• gdown support for downloading examples added as mentioned in #9
• [x] some comparison to mpas-tools grid-to-triangles Triangulation
• some doc updates

[!NOTE] Should add dask-core and gdown to the conda-forge recipe for this release.

[knubez]

Directory for sample MPAS native data:

/glade/derecho/scratch/knocasio/FROM_CHEYENNE/15km_MPAS_IFS_8sept_12utc_Thompson_MYNN/diag.2006-09-*.nc"

Directory of mesh:

/glade/derecho/scratch/knocasio/FROM_CHEYENNE/15km_MPAS_model_IFS/mpas_15mesh.nc