MIXER: Now using a std::array to hold VCOs

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
SYNTHESIZER_COMMON: Added a fucntion to check if one integer is a multiple

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
MISC: Asserting everywhere, whenever possible

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
TESTS: Reducing tests duration in debug configuration

Signed-off-by: Gama <misept.dieseneuf@gmail.com>