Code Sample, a copy-pastable example if possible

I'm working on a proof-of-concept for the geoxarray project where I'd like to store coordinate reference system (CRS) information in the coordinates of a DataArray or Dataset object. I'd like to avoid subclassing objects and instead depend completely on xarray accessors to implement any utilities I need.

I'm having trouble deciding what the best place is for this CRS information so that it benefits the user; .coords made the most sense. My hope was that adding two DataArrays together with two different crs coordinates would cause an error, but found out that since crs is not a dimension it doesn't get treated the same way; even when changing join method to 'exact'.

```python from pyproj import CRS import xarray as xr import dask.array as da

crs1 = CRS.from_string('+proj=lcc +datum=WGS84 +lon_0=-95 +lat_0=25 +lat_1=25') crs2 = CRS.from_string('+proj=lcc +datum=WGS84 +lon_0=-95 +lat_0=35 +lat_1=35')

a = xr.DataArray(da.zeros((5, 5), chunks=2), dims=('y', 'x'), coords={'y': da.arange(1, 6, chunks=3), 'x': da.arange(2, 7, chunks=3), 'crs': crs1, 'test': 1, 'test2': 2})

b = xr.DataArray(da.zeros((5, 5), chunks=2), dims=('y', 'x'), coords={'y': da.arange(1, 6, chunks=3), 'x': da.arange(2, 7, chunks=3), 'crs': crs2, 'test': 2, 'test2': 2})

a + b

Results in:

<xarray.DataArray 'zeros-e5723e7f9121b7ac546f61c19dabe786' (y: 5, x: 5)>

dask.array<shape=(5, 5), dtype=float64, chunksize=(2, 2)>

Coordinates:

* y (y) int64 1 2 3 4 5

* x (x) int64 2 3 4 5 6

test2 int64 2

```

In the above code I was hoping that because the crs coordinates are different (lat_0 and lat_1 are different and crs1 != crs2) that I could get it to raise an exception.

Any ideas for how I might be able to accomplish something like this? I'm not an expert on xarray/pandas indexes, but could this be another possible solution?

Edit: xr.merge with compat='no_conflicts' does detect this difference.