Fitting in Linear Space

[ryanhammonds]

This issue is described here: https://github.com/rdgao/daily_notebook/blob/master/daily19_04_04_noisefloor.ipynb

In summary, when the aperiodic scale/sd becomes large, oscillations may be missed during the standard fitting procedure in semi-log space. A potential solution was described by @rdgao and my understanding is to remove the aperiodic fit in linear space (spectrum - 10**fm._ap_fit). The fitting procedure after the ap_fit is removed may also need to be updated to work in linear space. There is also the issue of low frequency noise I'm not sure how to account for.