merge indexed tables with aggregation

[yakir12]

These is a method like that for ndsparse tables:

merge(left::DNDSparse{I1,D1}, right::DNDSparse{I2,D2}; agg=IndexedTables.right)

but not for indexed tables.

As a side-note, there seem to be a bunch methods that work with one but not the other and some inconsistencies in the API between the two table types (for instance you can supply the indexcol as a tuple when loading an indexed table but it must be a vector when loading a ndsparse table).