Support/think about using MultiIndex columned dataframes in the whole workflow

[milesgranger]

Relates to #369

To supply multiple multiple aggregation methods per tag, it is a cleaner solution to use pd.MultiIndex for columns such that:

For two aggregation functions:

                    tag1     tag2    
                    mean sum mean sum
2016-01-01 00:00:00    4  36    4  36
2016-01-01 01:00:00    9   9    9   9

or if a single agg:

                    tag1 tag2    
                    mean mean
2016-01-01 00:00:00    4    4
2016-01-01 01:00:00    9    9

#369 has added support for the aggregation functions, but no support for the larger use of these multi level columns in the remainder of the project.