Remove need for STIR_REGISTRIES (better STIR plug-ins)

It might be better to remove the need for explicit linking of the registry files (see #211 and #83).

This seems to need a number of function like init_IO_registries, to be called at the start of every main(), or STIR library initialisation (e.g. for Python).

This is ugly however. Worse, we don't know what that function should do. A developer can currently extend STIR (using the STIR_local CMake variable) and add her own registry variables. This allows anyone to add new IO/filter/whatever functionality, used by the rest of STIR. Indeed, they can add new registries as well. So, we'd need to provide hooks for that.

This is of course standard plug-in stuff. I'm sure there are nice and easy ways to solve this. suggestions welcome.

Start by reading the related issues #211 and #83, then trace how STIR registry files are linked and where main() or library initialization occurs. Identify the requirements for extensions using STIR_local, including custom registries and Python initialization. Done means the plugin and registry approach is defined well enough to remove explicit registry linking without breaking existing extension points.