Refactor init to create in matrix.hh and vector.hh

[Robert-Mic]

As we have discussed, init may lead to a slight confusion between creation of matrix in database and actual filling of said matrix with values. As proposed, we should change the name of this function to create, as CREATE is used in cql inside the function.

This change is, however, put on hold, due to init being mentioned in our bachelors thesis.