prototype for a unified load function for all file types

This is a prototype of one way to do a unified interface like we talked about at SciPy. This is the simplest thing I could think of to provide a unified load for all the file types that pims supports, it is almost certainly not ready to be merged, and may not be the right idea at all, but this code can serve as a starting point for a discussion of if this is even a good idea.
Should we keep this outside the function, somewhere in the module? And maybe call it `_sort_on_priority` so it is not exposed to the user?