Fix RK solver to avoid getting stuck when derivatives are estimated using finite difference (e.g. in the sine modulated population model). Introduce a min step allowed. TODO: Make it a parameter of the solver

git-svn-id: http://dev.eclipse.org/svnroot/technology/org.eclipse.stem/branches/STEM_1_4_0@3452 92a21009-5b66-0410-b83a-dc787c41c6e9