Implement xarray fieldlist

[sandorkertesz]

Fixes to #158.

Implements xarray fieldlists. Example:

>>> import earthkit.data
>>> import xarray as xr
>>> ds = earthkit.data.from_object(xr.open_dataset("docs/examples/tuv_pl.nc"))
>>> ds
<earthkit.data.readers.netcdf.XArrayFieldList object at 0x16059c490>
>>> ds.ls()
   variable  level                time    units
0         t   1000 2018-08-01 12:00:00        K
1         t    850 2018-08-01 12:00:00        K
2         t    700 2018-08-01 12:00:00        K
3         t    500 2018-08-01 12:00:00        K
4         t    400 2018-08-01 12:00:00        K
5         t    300 2018-08-01 12:00:00        K
6         u   1000 2018-08-01 12:00:00  m s**-1
7         u    850 2018-08-01 12:00:00  m s**-1
8         u    700 2018-08-01 12:00:00  m s**-1
9         u    500 2018-08-01 12:00:00  m s**-1
10        u    400 2018-08-01 12:00:00  m s**-1
11        u    300 2018-08-01 12:00:00  m s**-1
12        v   1000 2018-08-01 12:00:00  m s**-1
13        v    850 2018-08-01 12:00:00  m s**-1
14        v    700 2018-08-01 12:00:00  m s**-1
15        v    500 2018-08-01 12:00:00  m s**-1
16        v    400 2018-08-01 12:00:00  m s**-1
17        v    300 2018-08-01 12:00:00  m s**-1
>>> ds[0]
XArrayField(t,level=1000,time=2018-08-01 12:00:00)
>>> ds[0].metadata("level")
1000