Changed the namespace structure when UG is compiled as C++.

So far, there were three disjoint namespaces: UG, UG2d and UG3d.
Now, everything is contained in the namespace UG.  Additionally,
stuff that depends on the dimension is contained in the namespaces
D2 and D3 which are defined within UG.
This allows to put the user-defined types INT, SHORT, etc. into the
namespace UG without having to put namespace prefixes in front of
each and every occurrence of them in a header.  On the downside,
I cannot use END_NAMESPACE anymore to end a dimension-dependent
namespace.  Now there is a separate END_UGDIM_NAMESPACE for that.

[[Imported from SVN: r8182]]