Add function to grab surface values from profiles / all3d in compareAEM3D.py

[pclemins]

The surface value changes at each time step depending on the depth of the lake... so, one needs to find the last unmasked value for each time step and then grab that value for each time step.

See these links:

https://stackoverflow.com/questions/23435782/numpy-selecting-specific-column-index-per-row-by-using-a-list-of-indexes
https://stackoverflow.com/questions/17074422/select-one-element-in-each-row-of-a-numpy-array-by-column-indices