All-in-one include file for GUL14.

This file includes almost all other headers provided by the library. Two specific files are excluded, however:

gul14/catch.h: The header file for the Catch2 unit test framework
gul14/date.h: Howard Hinnant's header for calendar date functionality

While the unit test framework is simply not useful for inclusion into regular code, date.h is excluded chiefly because of its negative impact on compile times. Just include it directly where needed.