Exposing atomic_vs in python API.

Exposed a new function atomic_vs in the python API, which gives atomic surface and volume contributions. This release breaks backward compatibility of the fortran and C interfaces, but not for the python API. As such, the major version stays as it is (this is first and foremost a python-intended library).

Changes are otherwise minimal (extension of the test-suite, etc.).

First release

Fist release. All tests pass and performance seems much better than previous tests.