Clear waring in tests and benchmarks.

I would like to clear some warning that appears when compiling the
tests and benchmarks and improve code organization:

1 - add a macro named NOT_USED (in utils.hpp) to mark tell to the
compiler that a variable that has been assigned a value, it 'll not be
used.

#define NOT_USED(x) (void)(x)

matrix m, a, b;
m = a * b;
NOT_USED(m);

NOTE: some tests already avoid this warning.

2 - Remove unused parameters, specially to 'main' functions.

3 - Three tests uses the template function mean_square. This function
could be placed to a header file.

Joaquim Duran