Allow user-set BW's for MGWR

[TaylorOshan]

So right now we can set a min and max bw but it applies to all of the underlying gwr routines during the bw search and this means that we cannot set individual bw's for each covariate. For instance, we can force the bw min and max to be 20 but then we get a bandwidth of 20 for each covariate. It would be good to have a mechanism to individually set the bandwidth for each covariate, in case a user wants to manually explore relationships. I also needed to do this for the standard error simulations, so I came up with awkward hack but haven't yet figured out to best pull it through the API.

The one difference between gwr and mgwr is that I don't think we set the bw manually in mgwr the same way we can in gwr. In gwr, you can simply select the bandwidth. But in mgwr, you need the results of the bandwidth search procedure, like the partial residuals that will be unique to the combination of covariates, their starting bandwidth search values, and potentially the ordering of the variables (has anyone checked this out yet?). So I think it makes sense to have the manual bandwidth definition for mgwr be set as a vector of mins and maxs in the search procedure.

[TaylorOshan]

Addressed in #15