Bounding boxes vector should be smaller and contiguous

[fdrmrc]

The minimal change in #64 highlights something serious. In order to map points back from real to unit space, we should not go through the many calls of the Mapping class, but just use BoundingBox::real_to_unit(). Surprisingly, this increased the computational times.

The reason is that the std::vector of bounding boxes is long as the number of elements of the underlying triangulation. This should really be contiguous.

[fdrmrc]

Fixed by #66