The class tt(Interval) is a simple support class defining the lower and upper
bounds of an interval as tt(size_t) values. It uses tt(std::string) conversion
functions to convert values stored in text format in tt(std::string) arguments
to tt(size_t) values.

The class defines two tt(size_t) values (tt(d_lower) and tt(d_upper)) and
several factory members as well as members to access tt(d_lower) and
tt(d_upper)'s values.
