Removed exit() calls from the fitting code. Those were moved to the main functions trying to mimcl the original behavior.

Minor API changes:

* Some methods now return a bool value indicating success instead of being void.
* ModelObject::FitStatistics() returns a negative value on error.
* DESolver::Solve() returns -1 on error, and DESolver::bestEnergy becomes negative in this case.
* LevmarFit() returns -1 on error.