Accept lambda in methods with one obvious xarray argument

[max-sixty]

Branching from #3770

Here's the proposal: allow lambdas on methods where the primary argument is a single xarray object, and interpret lambas as though they'd be supplied in a pipe method followed by the current method.
Taking the example from the linked issue:

In [1]: import xarray as xr                                                                                                                                         

In [2]: import numpy as np                                                                                                                                          

In [3]: da = xr.DataArray(np.random.rand(2,3))                                                                                                                      

In [4]: da.where(da > 0.5)                                                                                                                                          
Out[4]: 
<xarray.DataArray (dim_0: 2, dim_1: 3)>
array([[       nan, 0.71442406,        nan],
       [0.55748705,        nan,        nan]])
Dimensions without coordinates: dim_0, dim_1

# this should be equivalent (currently not valid)

In [5]: da.where(lambda x: x > 0.5)

# the longer version (currently works)

In [5]: da.pipe(lambda x: x.where(x > 0.5))
                                                                                                                                 

Others I miss from pandas: assign, and loc.
I haven't gone through the list though assume there are others; we don't have to agree 100% on the list before starting with the most obvious ones, assuming we're in agreement with the principle.

[max-sixty]

assign is already supported! 🤦‍♂

[max-sixty]

where is complete

I think the others that could be worthwhile are the selection methods:

• sel & isel
• loc
• reindex?
  ...but much of their use case is bool arrays, which is mostly covered by where(..., drop=True).

Interested in what others think. I'm in no rush to implement those; where seemed much clearer to me.

[max-sixty]

Closing; can do others with the benefit of more experience