Support for different backends transfers

The normal behaviour of cargo is to transfer from one mpio capable backend (BEEGFS, LUSTRE, GPFS) to a POSIX filesystem (ideally an adhoc fs)

The adhoc FS communication is done using the posix calls (so LD_PRELOAD makes its magic) or directly using the API.

It could be a potential use case to be able to transfer between to posix backends.

To achieve that we need:

1. Finish sequential implementation (so two LDPRELOADS makes its magic)
2. Add support for input and output backend types.