- Renamed files ColDate* to ColTypeDate*.
- Renamed objects ColumnDate* to ColumnTypeDate*.
- This is mainly to make way for ColType{Duration,String,Numeric}, while leaving
ColString untouched. ColString is used only for ViewText columns, but may soon
be replaced by ColTypeString. Unknown.
- Refactored column objects to contain a ::validate method, for the
validation of incoming data.
- Context.columns is now a vector of one of each column object,
indexed by attribute name, for validation purposes.