Added Leapfrog explicit integrator.
To be checked because it should be symplectic, but looks like it is not..
Added Euler Implicit integrator, with Newton Raphson iterations etc.
It works ok.
Added trapezoidal implicit integrator, with NR iterations and 2nd order accuracy.
Tested. It works ok.
Added prototype of HHT implicit integrator.
NOTE: not working, it must be tested.
Better comments to code.
Various changes to the ChIntegrable classes
Updated tests about the ChTimestepper stuff