Add CHS Save Point hazard provider

[HenryGeorgist]

Is your feature request related to a problem? Please describe.
Save points are less dense than nodal data - adding a provider that handles this difference in density is necessary.

Describe the solution you'd like

1. add areas that relate to the save points to know which save point is valid for a structure
2. add ability to leverage nearest neighbor to set the save node for a structure.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[HenryGeorgist]

This should be in go-coastal, not go-consequences

[trietmnj]

This is being tracked through the CHART project dashboard - for CHART, this should be low priority until needed for the FET tool. Feel free to change the priority if there is a different reason or use case.