[FEA] Implement comparison operators for decimal columns

[ChrisJar]

Is your feature request related to a problem? Please describe.
I'd like to be able to perform boolean comparisons (>, >=, <, <=, ==) on dataframes and series with decimal columns.

Describe the solution you'd like
I would like to mimic the behavior of performing comparisons on dataframes or series with float columns. For example:

df = cudf.DataFrame({'id': [0, 1, 1], 'val': [1.00, 1.01, 1.02]})
df >= 1

returns

	id	val
0	False	True
1	True	True
2	True	True

But if a decimal column is present

df['val'] = cudf.Series([decimal.Decimal(x) for x in [1.00, 1.01, 1.02]], dtype=cudf.Decimal64Dtype(7,3))
df >= 1

it returns

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-31-5b213dd19a01> in <module>
----> 1 df >= 1

/home/u00u7rh1e72hXfsipJ357/miniconda3/envs/rapids-gpu-bdb/lib/python3.7/site-packages/cudf/core/dataframe.py in __ge__(self, other)
   2420 
   2421     def __ge__(self, other):
-> 2422         return self._apply_op("__ge__", other)
   2423 
   2424     def __invert__(self):

/home/u00u7rh1e72hXfsipJ357/miniconda3/envs/rapids-gpu-bdb/lib/python3.7/site-packages/cudf/core/dataframe.py in _apply_op(self, fn, other, fill_value)
   1470         ):
   1471             for col in self._data:
-> 1472                 result[col] = op(self[col], other)
   1473         else:
   1474             raise NotImplementedError(

/home/u00u7rh1e72hXfsipJ357/miniconda3/envs/rapids-gpu-bdb/lib/python3.7/site-packages/cudf/core/dataframe.py in op(lhs, rhs)
   1399         def op(lhs, rhs):
   1400             if fill_value is None:
-> 1401                 return getattr(lhs, fn)(rhs)
   1402             else:
   1403                 return getattr(lhs, fn)(rhs, fill_value)

/home/u00u7rh1e72hXfsipJ357/miniconda3/envs/rapids-gpu-bdb/lib/python3.7/site-packages/cudf/core/series.py in __ge__(self, other)
   1659 
   1660     def __ge__(self, other):
-> 1661         return self._binaryop(other, "ge")
   1662 
   1663     def __invert__(self):

/home/u00u7rh1e72hXfsipJ357/miniconda3/envs/rapids-gpu-bdb/lib/python3.7/contextlib.py in inner(*args, **kwds)
     72         def inner(*args, **kwds):
     73             with self._recreate_cm():
---> 74                 return func(*args, **kwds)
     75         return inner
     76 

/home/u00u7rh1e72hXfsipJ357/miniconda3/envs/rapids-gpu-bdb/lib/python3.7/site-packages/cudf/core/series.py in _binaryop(self, other, fn, fill_value, reflect)
   1158         else:
   1159             lhs, rhs = self, other
-> 1160         rhs = self._normalize_binop_value(rhs)
   1161 
   1162         if fn == "truediv":

/home/u00u7rh1e72hXfsipJ357/miniconda3/envs/rapids-gpu-bdb/lib/python3.7/site-packages/cudf/core/series.py in _normalize_binop_value(self, other)
   1551             return cudf.Scalar(other, dtype=self.dtype)
   1552         else:
-> 1553             return self._column.normalize_binop_value(other)
   1554 
   1555     def eq(self, other, fill_value=None, axis=0):

AttributeError: 'DecimalColumn' object has no attribute 'normalize_binop_value'

[harrism]

I this this already works in libcudf. This is just a Python feature request, right?