proj: update to 5.1.0

See NEWS for full changelog.  Besides Bug fixes, notable changes are:

o Function proj_errno_string() added to proj.h API (#847)

o Validate units between pipeline steps and ensure transformation
  sanity (#906)

o Print help when calling cct and gie without arguments (#907)

o CITATION file added to source distribution (#914)

o Webmercator operation added (#925)

o Enhanced numerical precision of forward spherical Mercator near
  the Equator (#928)

o Added --skip-lines option to cct (#923)

o Consistently return NaN values on NaN input (#949)

o Removed unused src/org_proj4_Projections.h file (#956)

o Java Native Interface bindings updated (#957, #969)

o Horizontal and vertical gridshift operations extended to
  the temporal domain (#1015)

(gdt)