enforce is a function comparable to assert(), it ensures that a condition is
true. However, while assert() should be used for conditions where we can be
certain that they are true, enforce is intended to be used for conditions that
can fail (like on invalid input). Therefore enforce() throws an exception when
the condition is false and does not abort the program's execution. Also, it is
not turned off by defining NDEBUG.
- add a new file enforce.hpp
- implement enforce()