Make it possible to load data from a file rather than a code object in the (new) photon emitter

[andrewgait]

It is currently only possible to load a code object (currently a Mitsuba VolumeGrid object) in as (a list of) photons within a Mitsuba scene. As with other scene objects (see e.g. src/emitters/envmap.cpp, src/textures/bitmap.cpp, and so on) it should also be possible to load these in from a file via XML as well (it is obviously (?) not possible to load a "runtime" code object in via an XML file).