Fix warning about log-linear frequency spacing

[rossjjennings]

In PR #1877, @vhaasteren introduced a new log-linear frequency spacing mode. Unfortunately, some of the associated code gives rise to a bunch of spurious warnings of the form

WARNING (pint.models.noise_model:1319): Log-spaced parameters are ignored because logmode, nlog,
and f_min ALL neeed to be setUse: logmode > 0 and nlog > 0 and f_min > 0.

when not using the new feature (i.e., using purely linearly-spaced frequencies, corresponding to logmode=0). These show up because pint.models.noise_model.get_rednoise_freqs() is passed a positive value of f_min by default, regardless of logmode. Because, with logmode=0 and nlog=None, any value of f_min other than 0 or None triggers the warning, it shows up even under the default settings.

This PR changes the conditions for triggering the warning so that it doesn't show up in the case where f_min is nonzero but logmode=0 and nlog=None, which is the default when not using log-spaced frequencies. This is safe because f_min is ignored in this case anyway. I've also edited the warning message to hopefully make it more helpful if it does appear.

[vhaasteren]

Looks good to me, this is ready for merge

[vhaasteren]

Thanks for noticing this @rossjjennings . Those warnings were not covered by the unit tests, and strictly speaking nothing breaks, so that slipped through. Ready to merge

[dlakaplan]

I can merge this whenever you are happy.