Simpler atomic step

Last week Walid, Michal and Jan worked on a simplified version of the atomic step, i.e. the computation of an enclosure for a hybrid ODE-IVP over a fixed time segment. Jan implemented the specification and wrapped it in a function that makes it work as a drop-in replacement for the old atomic step, a.k.a. solve-VtE. Currently, the new atomic step produces a single constant enclosure for each variable, i.e. it cannot represent non-contiguous enclosures, such as those arising from discrete transitions in the simulation. This limitation can be lifted by changing the representation of the system state used by the new atomic step, from a single interval per variable and mode, to a set of intervals per variable and mode.