NetCDF Projection Handling and EuroCordex test

[dblodgett-usgs]

This PR includes:

1. Some further cleanup of NetCDF dimension, coordinate variable, and canonical axis order handling.
2. Coordinate reference system lookup and assignment.

I've made some opinionated choices in here but I think they are reasonable.

1. Something that was already here when I showed up was using the first variable as a representative variable for the coordinate system to be inserted into the stars dimension object. I've instantiated that with the rep_var variable.
2. In .get_nc_projection, I return WGS84 (as EPSG:4326) if no CRS is declared but the coordinate variable units are degrees. A warning is issued in this case.
3. If a NetCDF file is found to have 1D auxiliary coordinate variables (in addition to 2D lat/lon coordinate variables) they are used in preference to the 2D variables. I should probably only do this in the case that a CRS is available. Will implement in a moment and add to this PR.

I think we can say that, with this PR, #89 is fixed.

[edzer]

if the try() fails here, ret will be of class try-error. Is that what you intent to return?

[dblodgett-usgs]

Ahh. No. Will fix.

[edzer]

If I understand you correctly: for raster files that have a CRS that can be understood, we return the raster with appropriate CRS; for files without understandable CRS but with long/lat rasters, we return them as curvilinear grids.

My request: can we also make this optional, so that in the first case we can get the curviilinear long/lat raster returned? This may be helpful to understand the intention of the creators. Or is this already optional right now with using the curvilinear argument?

[dblodgett-usgs]

I think this is a good idea. There are potentially three options here.

1. use curvilinear (2D) coordinate variables because a user specifies them with the curvilinear argument.
2. use curvilinear coordinate variables but have stars figure out what the coordinate variable names are.
3. use 1d auxiliary coordinate variables and have stars figure out what the coordinate variables names are.

I'll make sure that option 1 occurs if the curvilinear input is supplied.

As an unrelated side note - I expect much of the functionality that is being factored out of the main read_stars function to find a home in ncmeta with specific unit tests. It's just more convenient to do all the refactoring here.

[edzer]

I'll work on rewriting read_stars to spawn, by default, into read_ncdf if the object(s) are NetCDF files, with an option to override this & go the GDAL path.

[dblodgett-usgs]

I'll add more test coverage and clean this up a bit later on. Don't merge yet.

[dblodgett-usgs]

@edzer and @mdsumner I think this is ready to merge if you are happy with it.

[edzer]

Great work!

[adrfantini]

Awesome step forward, cant wait to have some time to test this out!