v2.6.1: cMaBoSS overhaul, Fix bug with missing timesteps, Added custom PopMaBoSS output

• cMaBoSS overhaul : improved structure, added many methods to modify MaBoSS and PopMaBoSS simulation objects.
• Fix bug which skipped timesteps at the end of simulations
• Added custom output for PopMaBoSS, allowing to cumulate directly on simple metrics.
• Added durations to Observed graphs.