Added checking of drifter z-locations.
To keep things simple for now, I only check whether the initial z0
locations provided are within 1m of the bottom. I do this by making
an interpolation function for water depth, and checking the water
depth at the provided (lon0, lat0) locations.
added more help text to check_points function
print info on how many drifters were NaN-ed out
Removed arbitrary 1m above the bottom check. I had trouble with prepare_for_model_run not working because the particle was just below the bottom even after running the check. But that problem doesn't happen anymore. So, make the check cleaner until I run into the problem again.
Merge pull request #9 from dcherian/check_points_pr

Check points