Added sortby to target grid

[kjdoore]

When regridding using the conservative and most_common methods, the plane coordinates (i.e., latitude and longitude) needed to be in increasing order in the target grid. This adds sorting of the target grid.

Fixes the issue in #28.

[BSchilperoort]

Thanks for submitting this PR!

It's good to fix this bug, but I feel like it might be best to return the grid in the original order. This is how the interp based methods currently work.

The extra steps for this would be to compute the sorted grid, regrid using the sorted grid, and then sort by the original target_grid again:

for coord in target_grid.coords:
    data = data.sortby(target_grid[coord])

Could you also add tests for these cases (the monotonic increasing and monotonic decreasing coordinates)?

[kjdoore]

Good call! Yes, I'd be happy to make this adjustment and add some test to confirm it is working as intended.

[kjdoore]

@BSchilperoort, I made an update to return the regridded data in the original order of the target grid. sortby did not quite work as you proposed, but I was able to create a solution. Feel free to give it a review and let me know what you think.

[BSchilperoort]

Thanks for making the changes! I looked in xarray's documentation and code, and found the reindex_like method which seems to work very well and is just a single line.

I also reordered the tests a bit with more parameterization to make them more compact :)

The test suite has become a bit slow though, which is because we use a full ERA5 file for the tests. This is good for some of the bigger tests, but for things like the attributes and coordinates it is probably better to use a smaller file.

[kjdoore]

Great thanks! reindex_like seems like the way to go. I looked for something like that, but didn't find it on my pass through the xarray docs. I'm glad you did.