Simplifications and performance improvements

[dcmoura]

After running a profiler did some changes and simplifications.

Highlights:

• Use of flat lists for rows (_values is now a list instead of a list of lists)
• Input over a single iterator (instead of list of iterators)
• Column names are replaced by direct access to the _values list in the SQL expression (instead of instantiating variables with the column names for each row) - part of the processing of the SQL expression had to move to the processor class
• One evaluation per line (instead of one per column)

[recharte]

Good job on removing the nested lists in the processor is far more intuitive this way.

[dcmoura]

Yes, I also agree, thanks! Tried to push most of that logic into pre-processing.