Proposal: Enable Optional Parallelism in PyGeoProcessing

[phargogh]

This is a proposal to enable optional concurrent.futures.Executor-based parallelism in pygeoprocessing functions that are likely to be CPU-bound. Details are in this google doc:

https://docs.google.com/document/d/124-k3EFMozO2x20Fo0rI3_XOPxXucG18_RVEXFdsAfg/edit

[phargogh]

We talked about this in the software team meeting 2024-03-05 and had a few suggestions come up as we iterate on this proposal:

• Add some example scripts demonstrating the various use cases
  • To better understand the tradeoffs in API decisions, for align_and_resize_raster_stack in particular, elaborate on the case of breaking out warp_raster calls to separate taskgraph tasks compared with the proposed approach
• Describe where documentation would be added to describe the new features of this proposed functionality