Reorder README feature matrix by GIS workflow frequency

Sections now go from most-used to least-used in a typical workflow:

1. GeoTIFF / COG I/O (read your data first)
2. Surface (slope, aspect, hillshade -- the basics)
3. Hydrology (flow direction, accumulation, watersheds)
4. Flood (downstream from hydrology)
5. Multispectral (satellite imagery)
6. Classification (binning results)
7. Focal (neighborhood analysis)
8. Proximity (distance)
9. Zonal (zonal stats)
10. Reproject / Merge
11. Interpolation
12. Morphological
13. Fire
14. Raster / Vector Conversion
15. Utilities
16. Multivariate, Pathfinding, Diffusion, Dasymetric (specialized)

Previously alphabetical, which put Classification first and buried
Surface and Hydrology in the middle.

Author: brendancol