Undo the change of location of PCM (it does not work)
Clarification of offset direction, object type (ROOT-5597 and ROOT-5840).

For veritual inhertiance, the type of the object was ambiguous; it can be
either derived (as commonly used for ROOT and thus default) or base.
The latter is used by PyROOT's automatic upcast. The offset functions now
allow to specify the type (derived / base) of the object that is passed in.

"Derived" and "base" do not clarify the direction of the offset; they have
been replaced by "fromDerived" and "toBase" to signal the offset's sign.
Silence "set but not used" warnings.
root with '-l' (and '-b') never creates a splash-screen (thus a call to InitCocoa is skipped

and root hangs on exit). Thanks to Olivier for finding this bug.
Tell Offset() that the address is that of a base object (ROOT-5840).