Support file chaining

[tkittel]

In order to make life easier for users and hook-writers, it would be great if we could support automatic chaining of input files, in the sense that users provide a list of files (could support wild cards), and then simply loop over particles in all of those files.

It is probably simplest to ignore meta-data in this case and simply provide a few dedicated functions for setting up a new mcpl_inputchain_t object, which can be opened and closed with a dedicated function and looped through with a new mcpl_chainread function (perhaps a few other functions should exists as well, for forward skipping, rewinding and a way to tell the current file and position in the current file - for debugging/logging purposes).