Added I/O plugin functionality

now working:
each time checkpoint-restart for all I/O modes (POSIX, MPI I/O and SIONlib)

[x] head = 0 keep_last_ckpt = 0
[x] head = 0 keep_last_ckpt = 1
[x] head = 1 keep_last_ckpt = 0
[x] head = 1 keep_last_ckpt = 1

TODO
[x] different checkpoint sizes for SIONlib and MPI-I/O

TOFIX
There might be an issue with Reed-Solomon. Restart is not possible for the case that even ranks have different ckpt size as odd ranks and head is enabled.

test case:
head = 1
node_size = 8
group_size = 4