Also set the UnitCell whit the set_cell method
Fix the UnitCell constructors
Add == operator for UnitCell
Add tests for the Distances module
Fix the Distances module tests
Fix the distances computations for Orthorombic boxes

This is the end of a 3 week long bug seeking \o/
Fix the forces and energy computations
Test for Energy conservation in simulations
Fix RDF computation.

Why does this only fails now ?