Merge pull request #152 from lordgeorg/groupby_ops

adding Operations to GroupByScan
* >as< to specify the name of the resulting aggregation column
* >min< and >max< as new operators
* >distinct< option for count operator (naive distinct algorithm)
* unit test for empty tables
adds function for parameterized loading of (json) files
allows SortScan to order by column name (and still by column number)
adds function for parameterized loading of (json) files
allows SortScan to order by column name (and still by column number)