Potentially add interface to set parameter bounds for osr.

In principle, there is everything in place to use constrained optimizers with osr. An example on how to use this with a user-defined optimizer wrapper is at http://www.dynare.org/phpBB3/viewtopic.php?f=1&t=6835 However, we do not provide an interface to set such bounds. We should discuss if we should leave it at that (more work for users) or whether we provide functionality to automatically set the bounds (including their correct ordering in the parameter vector).