Add Geoprocessing Service

[brendancol]

• [x] Setup PR to track progress
• [x] Add integration test with test graph
• [x] implement graph service
• [x] make graph service enabled by default when server starts
• [x] add documentation
• [ ] add example notebook
• [ ] add xarray-spatial proximity as valid graph operation
• [ ] add xarray-spatial reclassify as valid graph operation
• [ ]