Avoid polluting Goalie namespace

[jwallwork23]

Closes #270.

This PR ensures all modules define __all__ and stops goalie.__init__ from importing anything outside of Goalie itself.

Only one code change is required, for an from firedrake.__future__ import interpolate import. Everything else is just formatting.

[ddundo]

Thanks! It would be neater to use Function(fs).interpolate(...) rather than assemble(firedrake.__future__interpolate(...)) but that's for another PR I guess :)

[jwallwork23]

Thanks! It would be neater to use Function(fs).interpolate(...) rather than assemble(firedrake.__future__interpolate(...)) but that's for another PR I guess :)

Opened #272.