Reimplement the computation of index, stride and iteration of coords.

The previous code manually unrolls loops, which makes very hard to maintain.
Simple loops are used instead to improve readability.
Fix `MultiArray.hpp` as a side-effect and add unit tests.
Merge branch 'maint-improve-locator-unit-tests' of github.com:DO-CV/DO-CV into maint-improve-locator-unit-tests

Conflicts:
	test/Core/test_Locator.cpp