error helps for derived columns

[nsheff]

if a user forgets to put a column in the 'derived column list', looper should prompt.

we can figure that out by just checking to see if the values in the column are listed as data sources. if so, we suggest adding column x to the derived column list.

[nsheff]

I think this could possibly be done in peppy at the eido schema validation step...

[nsheff]

No... the number of attributes should be unrelated to the number of sources.

I'm trying to say something different. This is talking about a column in the table whose values are listed in the 'sources' list, but whose column name is not listed as an attribute. it's a much more complicated check...