Disk files indexing

This is a spin off of #428 (closed) by @elena

Yes it is related, @alainm : actually when you start a simulation with for example: TimeStep 0.6283185307179586 modulo 10

disk0.h5 has physical time 0 ok!

disk10.h5 has physical time 6.9115 so 11 steps of 0.6283185307179586 have been done

disk20.h5 has physical time 13.1947 : now 13.1947-6.9115=6.2831 ... = 10 steps of 0.6283185307179586

at the restart the first output contains 11 steps , the following ones 10

it is probably an issue with < or <= somewhere but I do not know where ... I try to have a look, suggestions are welcome