Fix various errors in network connections

[jnettels]

This PR attempts to resolve #127 and the various problems described in it.
No function interfaces are changed and I think this could be considered a "hotfix" to v0.0.3.

Improve 'weld_segments' function

• Replace some for-loops with vectorized functions for better performance. The recursive design is still quite slow, but the change showed a 4x speed improvement in one test case.
• Fix an error that could remove lines unintentionally

Add plot for investigation of node-mismatch errors

• This helps identify where the error is located, e.g. when a network consists of unconnected components

Update 'boundary' line connection method

• Catch special cases where the regular street lines already touch the building. This would already fulfill the boundary method, but for dhnx a connection line is required for each building. Use midpoint for these cases.
• Remove a misplaced 'convex_hull' call

Implement a new function 'drop_parallel_lines'

• If multiple lines connect the same two points in a network, eomof.solph will throw an error. Keep only the shortest of all lines connecting the same two points.
• Since this is called only after all building connections have been created, I think it is save to remove those duplicate lines. There should be no circumstances where the solver chooses a longer line over a shorter one.

Prevent OSError exception in unary_union()

• This issue was already discussed in Access violation error in unary_union() with shapely and pygeos installed #107 and fixed for connect_points.py in b25921f. Now the same access violation error came up at this point. It could be confirmed that the issue happens only with shapely < 2.0 and is not occuring with shapely >= 2.0.
• If that ever becomes a requirement, the 'list()' could be removed.