Matrix Permanent Using Recursion

Computes the permanent of a matrix using recursion. The technique is known as "expansion by minors" or the Laplace expansion. Two versions are included:
1) The MATLAB language routine permanent_mat() is about 8 times faster than equivalent native MATLAB function by Xu plus it has some optimization for sparse matrices.

2) The C language routine permanent()uses the CMEX interface to integrate into MATLAB. It is more than 500 times faster than the native MATLAB function by Xu. Also, I have found it to be faster than more advanced algorithms when the matrix is very sparse. One optimization available in C is that the matrix is kept in-place, in memory. Thus, less memory in consumed and less time is spent copying the matrix.