Skip first frame in replay mode

[pjuda]

Currently in the replay mode first step is initialized from the init file and then the first frame of the replay is taken into account. While it is not a bug but expected behaviour, it would be useful to be able to skip the first frame of the replay, since it is frequently the same as the initial position.

A following approach is possible: add a flag in the input file while reading the replay file, like skip_first=True, which would ask i-PI to ignore the first frame and continue immediately to the second.