Examine usage of XPathIo and EXV_XPATH_MEMIO

[piponazo]

The class XPathIo seems to have different implementations depending on EXV_XPATH_MEMIO. However that definition is hard-coded in the file basicio.hpp:39.

We should investigate:

• if it makes sense to have both implementations in the codebase.
• In the last years we have been using the XPathIo which is inhering from FileIO. Could we obtain any benefit from using the MemIO one?
• That class seems to be only used in ImageFactory::createIo.
• In case we want to keep both implementations, we should at least make possible to choose which implementation to use at configuration time (Via CMake options).

[clanmills]

I don't remember anything about this class. It may have been something that @vok added to deal with extended attributes on the Mac.