allow explit/implicit tag assignement in GEO_Internals (same as in OCC_Internals). Implicit is now only for tag < 0 (and not tag <= 0): this is for backward compatibility, because quite a few models define entities "0".

GEO_Internals::synchronize should delete all entities in GModel that do not exist anymore in the internal data (similar to what is being done in OCC_Internals)

* modified new internal CAD APIs to always preserve the order of the entities in transformations (even across dimensions)
* refactored extrusions - the refactoring of the old Geo code is complete (the parser does not include Geo.h anymore!)
* fixed incoherence when deleting internal entities during boolean operations, after synchronzing them with GModel
* made sure we don't bind OCC entities to multiple tags

better synchronization of internal GEO model: it can now handle changing internal GEO entities (e.g. due to Coherence)

- added missing tags in some of the derived levelset classes
- when no valid tag is given, automatically create a new one

all the tests in benchmarks/levelsets run; let me know if this change impacts others

refactored Duplicata operation + moved Boundary/CombinedBoundary calculation to GModel: this allows to compute the boundary of hybrid CAD models

refactoring + removed periodic mesh specification from GEO_Internals : we now directly sync and use GModel

this is the way we should/will treat all extra tag-based attributes, not directly linked to a specific CAD kernel