Network refinement helper

[johnlees]

For network refinement, some efficient code which:

• Takes a (sorted) array of boundary positions.
• For each boundary position, gives the extra included edges as a list of (i, j).

These could then be added in sequence in network refinement which might make it faster.

Perhaps the fastest way to do this would be to:

• Sort the values by their distance from the first boundary.
• Iterate through until next distance exceeded.
• While iterating, pass original distance index through lookup functions to get (i, j).

Or it may be easier to simply apply the inside boundary function at every array value, and then map the passing distance indices through the lookup functions.

[johnlees]

Closed by #48