Avoid groupby.agg(callable) in groupby-var

[mrocklin]

This has two benefits

1. It's much faster the following benchmark shows a 5x improvement
2. It doesn't require the pandas-like container to implement
   groupby.agg(callable), which helps cudf

Benchmark

I get five-ish seconds for this on master
And less than one second on this branch

from time import time
import dask
df = dask.datasets.timeseries(dtypes={'id': int, 'data': float}).persist()

start = time()
for i in range(3):
    df.groupby('id').data.std().compute()
stop = time()

print(stop - start)`

• Tests added / passed
• Passes flake8 dask

cc @thomcom @TomAugspurger

[mrocklin]

@jangorecki this may affect your benchmarks

[thomcom]

Nice!