PHYSIKA_ASSERT && VECTORND && MATRIXMXN

1. Add PHYSIKA_ASSERT. Usage:
    (1) #include "Physika_Core/Utilities/physika_assert.h"
    (2) PHYSIKA_ASSERT(x)
Please use this customized assert instead of standard one in
development.

2. Add VectorND.
3. Add operations for MatrixMxN.
4. Add various asserts in code.