reliability model - better multi-drive failure formula
    correct modeling of failures among multiple drives
    from N * pfail(drive FIT) to pfail(N * drive FIT)
    (little change to results, but it was incorrect)

Signed-off-by: markkampe <mark.kampe@inktank.com>
reliability model - GUI support for up to 6 copies

Signed-off-by: markkampe <mark.kampe@inktank.com>