Determinant bug (det) fixed

Before the det function sometimes changed the original values of the input array/matrix (to the Identity Matrix). I fixed this problems and added tests to account for that. I additionally added some more tests and enforced the input to be an array or a matrix since determinants are not defined for other types. Also there was an "eye" function in the _det function which was not found, so I added the prefix "math" to it.