Remove bounds checking in analysis
Give a size hint for RDF calculations
Fix typo
Add basic tests for topology reading
Get masses before the velocities initialisation
Add an atom_type field in Topology

Topology.atoms is now an array of Int, pointing to atom_types.
The getindex and setindex! convenience methods are defined and allow
to add and retrieve Atom(s) in topology.
add_interaction method !
Move MDSimulation constructor to MolecularDynamics.jl
TrajectoryOutput output type
Use @__FILE__ instead of non exported function