Support multi-column specification for positive and negative constraint

[sarahmish]

Allow users to specify multiple columns in Positive and Negative constraints. For example, the user can specify one column as Positive(columns='a') or Positive(columns=['a', 'b']) for multi-column.

For this change, we can expand the GreaterThan constraint and propose the following API changes.

gt = GreaterThan(low='a', high='b', scalar=None)
gt = GreaterThan(low='a', high=10, scalar='high')
gt = GreaterThan(low=10, high='a', scalar='low')

By default make scalar=None.
This makes the user specifically assign whether they are using a scalar comparison or not. Internally, this change will help us pretendedly know what are the columns in the data are under the constraint.

[sarahmish]

Changes in PR #533
Closing