CFTime offsets missing for milli- and micro-seconds

[aulemahal]

The smallest cftime offset defined in xarray.coding.cftime_offsets.py is "second" (S), but the precision of cftime objects goes down to the millisecond (L) and microsecond (U). They should be easily added.

PR #4033 adds a xr.infer_freq that supports the two, but they are currently untested as xr.cftime_range cannot generate an index.

MCVE Code Sample

xr.cftime_range("2000-01-01", periods=3, freq='10L')

Expected Output

CFTimeIndex([2000-01-01 00:00:00, 2000-01-01 00:00:00.010000,
             2000-01-01 00:00:00.020000],
            dtype='object')

Problem Description

An error gets raised : ValueError: Invalid frequency string provided .

Versions

Output of xr.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.8.2 | packaged by conda-forge | (default, Apr 24 2020, 08:20:52)
[GCC 7.3.0]
python-bits: 64
OS: Linux
OS-release: 5.6.13-arch1-1
machine: x86_64
processor:
byteorder: little
LC_ALL: None
LANG: fr_CA.utf8
LOCALE: fr_CA.UTF-8
libhdf5: 1.10.5
libnetcdf: 4.7.4

xarray: 0.15.2.dev9+g6378a711.d20200505
pandas: 1.0.3
numpy: 1.18.4
scipy: 1.4.1
netCDF4: 1.5.3
pydap: None
h5netcdf: None
h5py: 2.10.0
Nio: None
zarr: None
cftime: 1.1.1.2
nc_time_axis: 1.2.0
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: 1.3.2
dask: 2.16.0
distributed: 2.16.0
matplotlib: 3.2.1
cartopy: None
seaborn: None
numbagg: None
pint: 0.11
setuptools: 46.1.3.post20200325
pip: 20.0.2
conda: None
pytest: 5.4.1
IPython: 7.13.0
sphinx: 3.0.2