Neighborless observations: No weights with 0 connections and add flag

[jkoschinsky]

Right now you can create a weights file that has all zero connections (the NYC data with queen weights) without any warning. If people aren't paying attention (e.g. checking weights histogram), they could easily load it and compute Moran’s I and get non-sensical results.

Let's prevent weights creation in cases with zero connections and add a flag any time there are neighborless observations (we might already have this flag in an inactive state):

Text for cases with zero connections for contiguity weights:
None of your observations have neighbors. This could be related to digitizing problems, which can be fixed by adjusting the precision threshold.

Text for cases with any zero connections (contiguity or distance weights):
There is at least one neighborless observation. Check the weights histogram and linked map to see if the islands are real or not. If not, adjust the distance threshold (points) or the precision threshold (polygons).

[jkoschinsky]

Verified in GeoDa 1.8.6 (OSX Yosemite, 10.10.5).