BUG: Allow group_by and order_by column as window input in pandas backend

[icexelloss]

What change is proposed?

This PR fixes a bug where group_by and order_by column cannot be used as window operation input columns. It used to throw an exception.

# This throws an exception before the patch

w = ibis.trailing_window(
    order_by="time",
    group_by="tid",
    preceding = 1000,
)
table['time'].count().over(w).execute()

Exception:

../generic.py:460: in execute_reduction_series_groupby
    return aggcontext.agg(data, type(op).__name__.lower())
../../aggcontext.py:489: in agg
    indexed_by_ordering = frame[columns].set_index(order_by)
/Users/icexelloss/miniconda3/envs/ibis-dev/lib/python3.7/site-packages/pandas/core/frame.py:4351: in set_index
    index = ensure_index_from_sequences(arrays, names)
/Users/icexelloss/miniconda3/envs/ibis-dev/lib/python3.7/site-packages/pandas/core/indexes/base.py:5288: in ensure_index_from_sequences
    return Index(sequences[0], name=names)
/Users/icexelloss/miniconda3/envs/ibis-dev/lib/python3.7/site-packages/pandas/core/indexes/base.py:390: in __new__
    return Int64Index(data, copy=copy, dtype=dtype, name=name)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

cls = <class 'pandas.core.indexes.numeric.Int64Index'>
data = array([[1, 1],
       [3, 3],
       [2, 2]]), dtype = None, copy = False
name = 'plain_int64'

    def __new__(cls, data=None, dtype=None, copy=False, name=None):
        cls._validate_dtype(dtype)

	# Coerce to ndarray if not already ndarray or Index
        if not isinstance(data, (np.ndarray, Index)):
            if is_scalar(data):
                raise cls._scalar_data_error(data)

            # other iterable of some kind
            if not isinstance(data, (ABCSeries, list, tuple)):
                data = list(data)

            data = np.asarray(data, dtype=dtype)

	if issubclass(data.dtype.type, str):
            cls._string_data_error(data)

	if copy or not is_dtype_equal(data.dtype, cls._default_dtype):
            subarr = np.array(data, dtype=cls._default_dtype, copy=copy)
            cls._assert_safe_casting(data, subarr)
        else:
            subarr = data

	if subarr.ndim > 1:
            # GH#13601, GH#20285, GH#27125
>           raise ValueError("Index data must be 1-dimension

This PR fixes this issue.

How is this tested

test_window_on_and_by_key_as_window_input is added

[jreback]

lgtm ping on green.

[jreback]

thanks @icexelloss