Fixed a bug in observables when particle numbers are not contiguous.
Merge branch 'master' of git://github.com/espressomd/espresso

Conflicts:
	src/tcl/statistics_observable_tcl.cpp
Inappropriate manner of applying the swimming velocity.
Minor modifications for swimming + force swimming.
TCL interface for ENGINE feature (not yet complete)
ENGINE feature: Add swimming velocity in the thermostat
ENGINE feature: Documentation for the new interface commands, TODO: Comment in stuff as soon as pushers and pullers get handled
Removed last traces of ignore_charges
Now also checking O3 if O5 is not possible (e.g. llvm)
particle print no longer checks for runtime errors, which cannot occur anyways
Removed the dysfunctional invalidate_system, updated checkpointing documentation.
Removed invalidate_system voodoo from tests
Updated NEWS.
Clarified docs on checkpointing.
Updated autotools files.
Missing cells_tcl.hpp
Merge branch 'master' of github.com:espressomd/espresso
Add derived features to code_info.
Fixed some warnings.
Removed compiler switch for HarmonicForce.