A signed-binary matrix is a matrix whose elements are either `-1`, `1` or `0`.
A zero-sum matrix is a matrix such that
the sum of each column and each row is zero.
For example, `M=[[-1,0,1],[0,0,0],[1,0,-1]]` is a zero-sum signed-binary matrix.
`a(n)` is the number of zero-sum signed-binary matrices of size `n\times n`.
It is easy to see that `a(1)=1` and `a(2)=3`.
Find `a(n)`.