Log Message:
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)