Removed unused cell_index
test - check that can actually open files.
Switched to a row-buffer. Added new out_full_row() method to builder.

I prefer to receive the parsed csv file one full row at a time,
with one larger row buffer.

I also do not need a virtual interface, as everything can be templated.

I kept the old virtual interface (as it doesn't 'cost' anything), and
I kept the old begin,cell(),end interface as it is useful in different
situations.