dataframe_for_reps_impl
=======================

Dataframe that constructs representations from the columns (and conversions between
them) when a dataframe is loaded. A keyed\_representation and a 
string\_representation
are constructed for each column, unless the column is made up only of numerical
values, in which case a vector\_representation is constructed.

column_rep
==========

	some_df column_rep column_name

Return a representation of the default type for the column `column_name`, if such
exists.  This is either the keyed_representation or the vector_representation.

	some_df column_rep column_name rep_type

Return a representation of subtype `rep_type` for the column `column_name`, if such
exists.

conversion
==========

	some_df conversion src_column tgt_column

Returns a dfr_lookup_conversion from the keyed_representation `src_column`
of `some_df` to the default representation of `tgt_column` of `some_df`.  A
numerical column may not be the source.