Fix NumPy loadtxt wrapper arguments.

The NumPy loadtxt keyword is called 'ndmin', not 'ndlim', so this
wrapper is used even in the new versions.
Use a non-empty array for testing np.loadtxt.

An empty array causes a warning from NumPy, and the result doesn't
really matter anyway.
Merge pull request #818 from QuLogic/fix-loadtxt-shim

Fix loadtxt shim
Manually check the source depth. Fixes #757.
Maximum depth actually depends on the model.

800 km works for both and should be plenty. It is non-trivial
to actually parse the model files to figure out the depth.