Sort Eigenvectors & Eigenvalues

Sort a set of eigenvectors and corresponding eigenvalues

This function takes in two matrices P and D, presumably the output
from Matlab's eig function, and then sorts the columns of P to
match the sorted columns of D (going from largest to smallest)

EXAMPLE:

D =
-90 0 0
0 -30 0
0 0 -60
P =
1 2 3
1 2 3
1 2 3

[P,D]=sortem(P,D)
P =
2 3 1
2 3 1
2 3 1
D =
-30 0 0
0 -60 0
0 0 -90