…TimeIndex

Conflicts:
	xarray/backends/netcdf3.py
	xarray/conventions/coding.py
	xarray/core/dataset.py

This is a first pass at the following:
- Resetting the logic for decoding datetimes such that `np.datetime64` objects
are never used for non-standard calendars
- Adding logic to use a `NetCDFTimeIndex` whenever `netcdftime.datetime`
objects are used in an array being cast as an index (so if one reads in a
Dataset from a netCDF file or creates one in Python, which is indexed by a time
coordinate that uses `netcdftime.datetime` objects a NetCDFTimeIndex will be
used rather than a generic object-based index)
- Adding logic to encode `netcdftime.datetime` objects when saving out to
netCDF files

These were related to a recent minor upstream change in pandas:
https://github.com/pandas-dev/pandas/blame/master/pandas/core/indexing.py#L1433

Conflicts:
	xarray/__init__.py

Conflicts:
	xarray/core/accessors.py
	xarray/core/common.py
	xarray/core/dataset.py

Conflicts:
	xarray/__init__.py
	xarray/backends/api.py
	xarray/backends/common.py
	xarray/backends/netcdf3.py
	xarray/conventions/coding.py
	xarray/convert.py
	xarray/core/dataset.py
	xarray/tests/test_coding.py
	xarray/tests/test_utils.py

Conflicts:
	xarray/coding/times.py

Conflicts:
	xarray/coding/times.py
	xarray/core/accessors.py
	xarray/core/dataset.py
	xarray/tests/test_coding_times.py
	xarray/tests/test_utils.py

…TimeIndex

Conflicts:
	xarray/core/dataset.py

…ime.datetime

Conflicts:
	xarray/core/common.py
	xarray/tests/test_coding_times.py

Prior to this, opening a dataset with enable_netcdftimeindex set to True
and then accessing one of its variables outside the context manager would
lead to it being decoded with the default enable_netcdftimeindex
(which is False).  This makes sure that lazy decoding takes into account
the context under which it was called.

Previously when concatenating variables indexed by a NetCDFTimeIndex
the index would be wrongly converted to a generic pd.Index

Conflicts:
	xarray/coding/times.py
	xarray/tests/test_coding_times.py

Also only allow for serialization of cftime.datetime objects when
using the standalone cftime package.

Add tests/fixes for dt accessor with cftime datetimes

are not available.  Use existing requires_cftime decorator where possible
(i.e. only on tests that are not parametrized via pytest.mark.parametrize)

…talled

Conflicts:
	xarray/backends/zarr.py

I must have inadvertently removed it during a merge.

…efault