Added W generation

Added KNN and threshold generated W objects on the network.  Updated ReadMe.

- KNN
    1. For the set of nodes V in the network compute Dijkstra's for all other nodes.  
    2. Use heapq.nsmallest to get the n smallest
    3. Remove the 0 distance source node
    4. Get the indices
    5. Pass to `pysal.W(neighbors, ids=ids)` to generate the W

- Threshold
    1. For the set of nodes, V, in the network compute Dijkstra's until we pop a node with distance greater than the threshold.
    2. Compute weights if necessary.
    3. Pass to `pysal.W(neighbors, weights, ids=ids)` to generate the W.

Notes
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- KNN equal distance breaks are arbitrary
- Distances are computed from the source node to the destination node.  Thresholding therefore requires traversal of the full edge to be included as a neighbor.