Fix compilation error in PeriodicFlow.hpp
Set python versions explicitly.
Move function declarations of _utils.cpp into .hpp

Also aabbExtrema() was moved into Shop.hpp
Fix runtime error in debug mode and without openmp.

See:
https://answers.launchpad.net/yade/+question/255699
getStress() considers now an adequate volume value for non-periodic case (values passed as parameters may still also be taken into account)
Re-introducing change from https://github.com/yade/trunk/commit/ab91b3cf8654f6d563dede10184e784a8ce84219, reverted since, without reason
Drop including "yade/" folder in cpp and hpp files.

Many years we had a symlink in build-dir "yade" which
points to a source dir. It was neccessary for scons build
system, which is not the case at the moment. All headers
must be included directly without "yade/" prefix, e.g.:

<core/Scene.hpp>
<pkg/common/Sphere.hpp>
Fix docs-generation.

Revert the previous behaviour, calling sphinx.main function.
It seems, new versions of sphinx are calling sys.exit() after
execution, which breaks our doc generation.

Now SystemExit is catched by try-except block.
#ifdef guard for openmp function
don't overwrite imposed fluid pressure when initializing the values (more flexible) + a function returning barycenter + improved "locate" for interpolation
increased flexibility of imposing fluid pressure in FlowEngine's
specialized getter/setter's for TwoPhaseFlowEngine + no re-indexing of cells in UnsatEngine
better #def/#ifdef logic for TwoPhaseFlowEngine
Merge github.com:yade/trunk
rename cellCenter->cellBarycenter (missing in prev. commit)
Merge github.com:yade/trunk
Improvement of https://github.com/yade/trunk/commit/05599b23808991dbfef6b85400c7ddd32be594e0 regarding volume computation in getStress for non periodic cases
-remove check cell.index
Merge branch 'master' of github.com:yade/trunk
comment #define TWOPHASEFLOW