Some memory savings in the Tesserae algorithm. (#9)

* Improvement: Only store values used in Tesserae.

* Tweak: Change the order of the value matrices to improve the possibilites for copying.

* Improvement: Implement a go at the LogSpace Algorithm from Zweig & Padmanabhan in Tesserae.

* Test tesserae with mem_limit.

* Bugfix: Handle the initialization-step properly in memory-reduced traceback.

* Tweak: Improving the tests as suggested in pull-request.

* Improvement: Getting rid of one unnessecary allocation in the main matrices.

* Tweak: Style improvements according to flake 8.

* Tweak: Style improvements according to flake 8 in tests.

Author: karljohanw