Fitting equidistant frequency points (as compared to other spacing)

[TomDonoghue]

As brought up by @rdgao :

Inputs PSDs currently require linearly spaced frequency points. This is not necessarily ideal, at least when thinking about fitting much broader frequency ranges.

Notes from Richard:
Fitting the Lorentzian (no knee) in semilog is mathematically equivalent to fitting a line in log-log (it still uses every frequency point). The specific issue is not from fitting in log-log, but in using linearly-spaced frequency points. The error between 10-20Hz weighs just as much as the error between 80-90Hz, which is potentially undesirable, given our a priori knowledge about brain PSDs.

For future version of FOOOF, we should consider:

• Is there anything fundamental about using linearly spaced frequencies? How can we, and should we think about moving to supporting different frequency scales. [As of right now, the algorithm definitely presumes even-spaced - it's an open question how fundamental this is].
• When and how do we want to accept, or even encourage different frequency spacing.

[TomDonoghue]

Following #154, this has been moved to:
fooof-tools/Development#3