For fit parameters, use sklearn trailing `_` notation?

[ljwolf]

Sklearn has standardized around using trailing underscores to denote quantities estimated from data.

I think this convention could be helpful? Sometimes, I'm not sure what quantities on a Sel_BW object versus a GWR object are estimated, especially since a gwr object receives a bw and does not, itself, estimate that bw.

[ljwolf]

standardizing around the trailing underscore would help identify what the "output" is for each class.