Allow `isel` to ignore missing dimensions?

[johnomotani]

Sometimes it would be nice for isel() to be able to ignore a dimension if it is missing in the Dataset/DataArray. E.g.

ds = Dataset()

ds.isel(t=0) # currently raises an exception

ds.isel(t=0, ignore_missing=True) # would be nice if this was allowed, just returning ds

For example, when writing a function can be called on variables with different combinations of dimensions.

I think it should be fairly easy to implement, just add the argument to the condition here

xarray/xarray/core/variable.py

Lines 1059 to 1062 in 65a5bff

	if invalid:
	raise ValueError(
	f"dimensions {invalid} do not exist. Expected one or more of {self.dims}"
	)

the only downside would be increased complexity of adding another argument to the API for an issue where a workaround is not hard (at least in the case I have at the moment), just a bit clumsy.