Hemispherical cap retreats

Introduce `CapVariable`, a hemispherical mask that retreats with the
advancing interface. Replaces `calc_hemispherical_cap` method.
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.
Add results for critical current

Results show the bump forming as the critical current is approached.
Merge branch 'issue19-retreating_hemisphere'