Introduce an RF frequency shift input to specify off-momentum computation

[lfarv]

Wherever there is an input for dp or dct to specify off-momentum computation, we can now alternatively use df, the RF frequency deviation from nominal. This off-momentum specification is closer to the way it is obtained in a real machine. So we can use one of these 3 keywords:

• dp=* (off-momentum)
• dct=* (path lengthening)
• df=* (RF frequency shift)

At this time, the following functions accept these inputs:

• atsetcavity
• findorbit(4D), findorbit4, findsyncorbit
• findm44
• atlinopt2, atlinopt4, atlinopt6
• tunechrom

Missing functions will be added later (atx, atsummary and ringpara are pending in #500)

[simoneliuzzo]

Dear Laurent,
I find this is a really nice addition.
I will not be able to review the pull request till January 2023. May be @carmignani could review the pull request instead of me?
all the best
Simone