Fix link to notebooks.
Nosetests works on all modules.

Nosetests works from the base directory on both scripts/ and
extremefill2D/ and on any file name.
Fix bug when the zero level set leaves domain.

When there is no zero level set the `getInterfaceHeight` can't find a
zero interface. This results in an `IndexError` when the `zeros`
variables is indexed. To overcome this, `getInterfaceHeight` just
returns the max value of y for the domain.
Notebook with constant_current17 simulations.