Gaussian elimination with partial pivoting.
GE(A)
An n by p matrix. n must be greater than or equal to p.
The output is a vector of length n.
A vector of length n. The first p elements are the indices of the pivoting elements, ordered accoridng to columns, and the rest n-p elements are the remaining indices from 1 to n.
This function performs Gaussian elimination to the input matrix and returns the locations of pivoting elements.