Add Support for Chained Arithmetics

[blaklaybul]

We currently only support arithmetics with 2 operands, i.e. the following throw an exception:

df['total_quantity'] * df['taxful_total_price'] * 0.65
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/michael/projects/eland/eland/series.py", line 678, in __mul__
    return self._numeric_op(right, _get_method_name())
  File "/Users/michael/projects/eland/eland/series.py", line 1039, in _numeric_op
    elif np.issubdtype(np.dtype(type(right)), np.number) and np.issubdtype(self._dtype, np.number):
  File "/Users/michael/projects/eland/eland/series.py", line 368, in _dtype
    return self._query_compiler.dtypes[0]
  File "/Users/michael/projects/eland/eland/query_compiler.py", line 104, in dtypes
    return self._mappings.dtypes(columns)
  File "/Users/michael/projects/eland/eland/mappings.py", line 501, in dtypes
    {key: np.dtype(self._source_field_pd_dtypes[key]) for key in field_names})
  File "/Users/michael/projects/eland/eland/mappings.py", line 501, in <dictcomp>
    {key: np.dtype(self._source_field_pd_dtypes[key]) for key in field_names})
KeyError: 'total_quantity___mul___taxful_total_price'

>>> df['customer_last_name'] + "," + df['customer_first_name']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/michael/projects/eland/eland/series.py", line 521, in __add__
    return self._numeric_op(right, _get_method_name())
  File "/Users/michael/projects/eland/eland/series.py", line 1014, in _numeric_op
    if (np.issubdtype(self._dtype, np.number) and np.issubdtype(right._dtype, np.number)):
  File "/Users/michael/projects/eland/eland/series.py", line 368, in _dtype
    return self._query_compiler.dtypes[0]
  File "/Users/michael/projects/eland/eland/query_compiler.py", line 104, in dtypes
    return self._mappings.dtypes(columns)
  File "/Users/michael/projects/eland/eland/mappings.py", line 501, in dtypes
    {key: np.dtype(self._source_field_pd_dtypes[key]) for key in field_names})
  File "/Users/michael/projects/eland/eland/mappings.py", line 501, in <dictcomp>
    {key: np.dtype(self._source_field_pd_dtypes[key]) for key in field_names})
KeyError: 'customer_last_name___add___,||str'

I think this is because of the way that we create intermediary field names to store the scripted fields. We'll need logic to understand that the latter operations rely on the first and make the appropriate field names available to subsequent ops.

[stevedodson]

Similarly aggs on chained series doesn't work.

[stevedodson]

Still moving this forward

[stevedodson]

Should be addressed by #103