option to snap to nearest *n* locations

[jGaboardi]

Currently, network.snapobservations() finds one single nearest location along the network to snap a point. As an option / improvement:

1. locate the n nearest locations along the network
2. add restricted links to the network on which a shortest path can enter/exit exactly twice
3. calculate network.allneighbordistances() from the update network in (2)

[jGaboardi]

Gaboardi, J.D., Folch, D.C. and Horner, M.W. (2019), Connecting Points to Spatial Networks: Effects on Discrete Optimization Models. Geogr Anal. doi:10.1111/gean.12211

[jGaboardi]

The is exactly related to #198 (pysal/pysal#650).