Moved core modules, rewrote third2oct

* Moved the modules inside core into the main namespace
* Rewrote esum. N-dimensionsal arrays are now supported. Acts like
np.sum()
* Rewrote third2oct. Now works for N-dimensional arrays. Performs the
summation along axis. If no axis is specified the highest dimensions is
used. Note that this might break backwards compatibility
* Renamed unittest folder to tests.

* Relative import for Python 3