Reorganize existing GEOS functions in ctypes_declarations.py

 - Add section headers from geos_c.h, and move functions to nearest
   position, and nest capabilities from each GEOS version in `if` blocks
 - No functions add (yet), only re-arranged order and verified GEOS version
 - Remove reference in LGEOS300 to GEOSUnionCascaded, which was
   introduced in GEOS 3.1.0