Error checking

Gandalf is written with a large amount of automatic error checking, for instance testing for accesses to illegal matrix or image data. When NDEBUG is defined, most of these tests are switched off, using a similar mechanism to assert(). It is up to the programmer to decide which tests are for bugs in the code, and can therefore be turned off when code is compiled for release by defining NDEBUG, and which are data-dependent errors and should therefore remain.