Add function checking if PyOP2 has been initialised
Add unit tests for initialised function
Update op2.init docstring
Introduce a compatible FFC version to check against

This prevents us from needlessly having to bump the compatible PyOP2
version in FFC whenever we bump the PyOP2 version in a way that does
not affect FFC compatibility.
Bump version to 0.4.0
Merge pull request #252 from OP2/initialized

Add function checking if PyOP2 has been initialised

op2.initialised returns True if op2.init has already been called and
op2.exit has not yet been called and False otherwise.

Since this changes the API the version is bumped to 0.4.0. To not require a
corresponding FFC version bump, introduce an FFC compatible version which
the ffc_interface checks against.