Problem: Given a matrix of m x n elements (m rows, n columns), return all elements of the matrix in spiral order.

For example,
Given the following matrix:

[
[ 1, 2, 3 ],
[ 4, 5, 6 ],
[ 7, 8, 9 ]
]


You should return [1,2,3,6,9,8,7,4,5].

Solution: Spiral Matrix, Python: