Snap dam to river channel

[changliao1025]

If a dam sits on a river channel, then it is possible that I need to insert a point to hold the dam:

[changliao1025]

This algorithm should be conducted in the following order:

1. find the edge that is the closest to the dam point;
2. find where the insert point should be
3. break the edge into two edges with the middle point as the new dam point
4. update the whole pyflowline object.

[changliao1025]

An example on the Columbia River:

[changliao1025]

It is possible that there is no need to break the edge since the dam is already on the edge line.
Adding a point does not force the mesh generator to place a cell at the point.