Simpler event handling

Last week Jan worked on improving the support for nonlinear invariants in Acumen's enclosure semantics and on alternative simulation time subdivision strategies. Michal and Jan also managed to simplify the event handling algorithm, by removing enclosures entirely from the event tree, so that it can be built up using vectors of intervals. The main benefit of this change, besides the simplification of the specification and implementation of event handling, is that the use of invariants to improve enclosures can now be easily described in terms of intersection of interval boxes. In the coming week Jan will make a formal analysis of the algorithm worked out for a simple example, and on a simplification of the solver algorithm specification.