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)

This is similar to what #501 introduced for Matlab

The following functions are modified to introduce the df keyword:

• find_orbit4, find_syncorbit, find_orbit
• find_m44
• get_revolution_frequency
• linopt2, linopt4, linopt6, get_optics, avlinopt
• get_tune, get_chrom

As a side effect, this PR also introduces new properties for the lattice object:

• cell_length
• cell_revolution_frequency
• cell_harmnumber

They are the 1-cell equivalent of the unchanged following properties related to the full ring (periodicity x cells):

• circumference
• revolution_frequency
• harmonic_number

[swhite2401]

Very useful, would it make sense to issue an error if 2 contradicting argument are provided. I saw you mention in the help that dct and df overwrite dp but is that sufficient?

[lfarv]

@swhite2401 : the test is added