new obstacle definition + doc relying on energy and enstrophy decay

Docs/energy_enstrophy_decay.pdf

Docs/energy_enstrophy_decay.tex

HySoP/hysop/domain/obstacle/__init__.py

HySoP/hysop/domain/obstacle/cylinder.py

HySoP/hysop/domain/obstacle/hemisphere.py

HySoP/hysop/domain/obstacle/obstacle.py

HySoP/hysop/domain/obstacle/obstacle_2D.py

HySoP/hysop/domain/obstacle/obstacle_3D.py

HySoP/hysop/domain/obstacle/sphere.py

HySoP/hysop/problem/navier_stokes.py

HySoP/hysop/unusedOrObsolet/obstacle_d_old.py

HySoP/hysop/unusedOrObsolet/obstacle_old.py