CalcCnErosRunoffSed.CalcCN index errors

There are a number of index errors in the CalcCnErosRunoffSed.CalcCN function.

Make sure these conditions are being met when trying to fix this issue, otherwise some parts of the code will not execute:

Line 30 in 6b9a3fd

if z.CN[l] > 0:

Line 102 in 6b9a3fd

if z.CNI[2, l] > 0:

Line 130 in 6b9a3fd

if z.CNP[2, l] > 0:

To get the model to run, I just commented out the call to this method in gwlfe.py.