Rewrite _find_get to handle bools and added defaults for freqfit

[amaliestokholm]

When during frequency fitting I discovered that it was required of me to specify threepoint=False in the freqparams xml tag.
That makes no sense as (i) that is for ratio fitting and (ii) it should be False by default.

I then discovered that _find_get could not handle the default value of threeport=False. This is a rewrite of that function.

While I was at it, I added defaults for fcor (cubicBG14), correlations (False), and dnufrac (0.15).
When fitting frequencies, you can simply just specify

<freqparams>
    <freqpath value={path-to-frequencies}
</freqparams>

and add <freqs/> to fitparams and maybe set

<freqplots>
    <True/>
</freqplots>.

[jakobmoss]

Merged by #18 .