Implement `value_counts` method

Implement value_counts method

MCVE Code Sample

print(object)
<xarray.DataArray (subtissue: 49, sample: 532, gene_id: 31490)>
dask.array<where, shape=(49, 532, 31490), dtype=object, chunksize=(1, 10, 31490), chunktype=numpy.ndarray>
Coordinates:
  * gene_id    (gene_id) object 'ENSG00000000003' ... 'ENSG00000285966'
  * sample     (sample) object 'GTEX-1117F' 'GTEX-111CU' ... 'GTEX-ZZPU'
  * subtissue  (subtissue) object 'Adipose - Subcutaneous' ... 'Whole Blood'

Suggested API:

object.value_count(**kwargs) should return an array with a new dimension defined by the kwargs key, containing the count values of all dimensions defined by the kwargs value.

Expected Output

object.value_count(observation_counts=["subtissue", "sample"])
<xarray.DataArray (observation_counts: 3, gene_id: 31490)>
dask.array<where, shape=(3, 31490), dtype=int, chunksize=(3, 31490), chunktype=numpy.ndarray>
Coordinates:
  * gene_id    (gene_id) object 'ENSG00000000003' ... 'ENSG00000285966'
  * observation_counts  (observation_counts) object 'underexpressed' 'normal' 'overexpressed'

Problem Description

Currently there is no existing equivalent to this method that I know in xarray.

Versions

Run the MCVE against current xarray and compare the requested value_counts API with the suggested arguments, dimensions, coordinates, and expected counts. Done means the API and method naming are resolved and the example produces the documented DataArray shape and values.