no way to specify prior on mode linewidth

[darthoctopus]

I'm currently trying to peakbag a Kepler star that is fairly evolved (Δν = 5 μHz), and the DR prior is yielding an unphysically large linewidth of >1μHz. There doesn't seem to be an easy way to override this short of manually editing the peakbag.priors object after instantiating the peakbag object, but before running peakbag (which isn't possible on the main branch). This also affects the modeID step.