Fit integer tune

[swhite2401]

This PR adds to possibility to fit the integer part of the tune either by set fit_integer=True as argument or providing a non-zero integer part for one of the target values.
Follow-up from #456

[simoneliuzzo]

Dear @swhite2401 , thank you for this addition. The function get_tune may be should have the same optional input?
for example, to check that the tune correction is correctly done I would do:

initial_tune = get_tune(ring, ..., use_integer = True)
ring_tune = at.matching.fit_tune(ring, ...,fit_integer=True)
final_tune = get_tune(ring_tune, ..., use_integer = True)

[simoneliuzzo]

Dear @swhite2401

the help of at.matching.globalfit.fit_tune gives examples that are may be not up to date.
The help of get_tune could be complemented with the get_integer optional flag

[simoneliuzzo]

works for me

[lfarv]

Works for me. Apart from the minor warnings I get (mentioned above), it's ok for me.