This PR allows strictly global bandwidths (inf) in both GWR/MGWR.

Users can force the bandwidth to be inf by calling:

GWR(coords, y, X, bw=np.inf, constant=True).fit()

The result will be exactly the same to OLS's.

In bandwidth selection users can allow to try global bandwidth by setting the bw_max = np.inf:

sel = Sel_BW(coords,y, X, constant=True)
bw = sel.search(verbose=True, bw_max=np.inf)

In MGWR, users can force some of the variables to have strictly global bandwidths by setting the minimum bandwidth searching for that covariate to be np.inf, for example.

sel = Sel_BW(coords,y,X,multi=True,constant=True)
bws = sel.search(verbose=True,multi_bw_min=[None,np.inf,np.inf])
#print(bws) -> [195,  inf , inf]

Or to allow the possibility of an inf bandwidth by setting the maximum bandwidth to be np.inf:

sel = Sel_BW(coords,y,X,multi=True,constant=True)
bws = sel.search(verbose=True,multi_bw_max=[None,np.inf,np.inf])
#print(bws) -> [70, 50, inf]

See this notebook for details.