Smaller default limit on max NR iterations for implicit solvers
Major change in API of the integrator: now the StateSolve() function is supposed to return dy/dt, whereas previously returned just  dy, and it was up to the user to divide by dt  if dy/dt was needed (this was meant to cope better with the DVI concepts,but also made things a bit confusing for a 'classical' approach to integration, so now we prefer to return dy/dt as for classical ODEs, and if one is interested to the finite Dy because it was a DVI, just multiply dydt by dt and get the Dy as computed by the DVI).
Fix one of the bugs that made HHT not working
Corrected signs in residual of HHT for case alpha != 0
This improves greatly the convergence of HHT. The M factor in Negrut paper (2007) is 1, but looks like it should be 1/(1+a) to be the correct jacobian. To be checked.
Various changes in timesteppers, mostly  to reflect the way StateSolve() is working in this last revision.
Various changes, mostly to reflect the new behavior of StateSolve() and StateSolveA()
Test the HHT integrator
Merge branch 'develop' of https://github.com/projectchrono/chrono into develop