Implement support for rotating RPPs

[amereghe]

The implementation should follow two paths:

1. forcing the use of $start_transform directives;
2. actually substituting an RPP with a boundle of 6 PLAs. This also imply modifygin on-the-fly the definition of the zones concerned by the body. In particular, each zone containing the -RPP expression should be replaced by six zones combining the substituting planes.