add type information to python representation of datahandles

TLDR: make type information available to the python representation of DataObjectHandle(Base).

With this MR, one can query a python'y GaudiKernel.DataObjectHandleBase to ask it which type of data object it holds via its novel member function type().
The C++'y DataObjectHandleBase doesn't hold a type itself, so the python type() results in "unknown_t".
But the derived DataObjectHandles do hold a type, and whenever a python DataObjectHandleBase is created from one of these derived handles, the type info is available, formatted according to System::typeinfoName.

@rmatev @sstahl @olupton @apearce fyi