Modular enclosure interpreter

Last week Jan factored out the time-subdivision strategy, event handlers IVP and solver algorithms from the enclosure interpreter. The restructuring of the interpreter infrastructure makes it easier to plug in alternative solutions for either of these components and, in particular, makes it possible to use them in any combination. Currently, the options available through the Semantics menu are to use the localizing strategy with either the PWL or event tree (EVT) event handling algorithms, with the option to use the Contraction (Lohner) IVP solver.