Add Rounding constraint

[katxiao]

Problem Description

To make the data look more realistic, we should round floats based on the user's specification. Sometimes, there are a set # of digits after the decimal. Other times, numbers might be to the nearest 100 or 1000.

Expected behavior

Add a Rounding constraint class that rounds floats based on the user's specification. Users should be able to specify the list of columns it needs to round, and how much to round each column.