Add USA geoid models 1999, 2003, 2006, 2009 and 2018

[rouault]

All of them are referenced, at least partially, by EPSG and/or LIDAR WKT1 CompoundCRS

[rouault]

@kbevers @hobu Does that look good to you ? After doing that, I realized that https://download.osgeo.org/proj/vdatum/ had .zip with previous grids. The names betwen those .zip and this pull requests are not always the same:

• I've tried to align on the name of the grids referenced by the EPSG dataset as much as possible. If they refer to geoid06_ak.bin, then I create a geoid06_ak.gtx (potentially merging the 4 NOAA .bin files together)
• For other files not referenced by EPSG, I keep the name of the upstream NOAA file and replace its .bin extension by .gtx

[kbevers]

It looks good to me. I haven't tested any of the grids but I trust that you have converted them correctly.

[rouault]

I haven't tested any of the grids but I trust that you have converted them correctly.

I trust the GDAL NGSGEOID driver that has been used recently to do the job
Also cross-checking on a random point among the various GEOID realizations and egm96_15.gtx shows consistency

For CONUS

$ gdallocationinfo -wgs84 ../egm96_15.gtx -77 40
Report:
  Location: (412P,200L)
  Band 1:
    Value: -34.5047569274902

$ gdallocationinfo -wgs84 g1999u03.gtx 283 40
Report:
  Location: (1140P,1080L)
  Band 1:
    Value: -33.820499420166

$ gdallocationinfo -wgs84 geoid03_conus.gtx 283 40
Report:
  Location: (3180P,1080L)
  Band 1:
    Value: -33.8349685668945

$ gdallocationinfo -wgs84 geoid09_conus.gtx 283 40
Report:
  Location: (3180P,1080L)
  Band 1:
    Value: -33.8376998901367

$ gdallocationinfo -wgs84 g2012bu0.gtx 283 40
Report:
  Location: (3180P,1080L)
  Band 1:
    Value: -33.8595008850098

$ gdallocationinfo -wgs84 g2018u0.gtx 283 40
Report:
  Location: (3180P,1080L)
  Band 1:
    Value: -33.880199432373

Alaska

$ gdallocationinfo -wgs84 ../egm96_15.gtx 175 65
Report:
  Location: (1420P,100L)
  Band 1:
    Value: 6.03143215179443

$ gdallocationinfo -wgs84 g1999a01.gtx  175 65
Report:
  Location: (180P,420L)
  Band 1:
    Value: 6.23759984970093

$ gdallocationinfo -wgs84 g2003a01.gtx  175 65
Report:
  Location: (180P,420L)
  Band 1:
    Value: 5.28455686569214

$ gdallocationinfo -wgs84 geoid06_ak.gtx  175 65
Report:
  Location: (180P,420L)
  Band 1:
    Value: 3.61653804779053

$ gdallocationinfo -wgs84 geoid09_ak.gtx  175 65
Report:
  Location: (180P,420L)
  Band 1:
    Value: 4.35279989242554

$ gdallocationinfo -wgs84 g2012ba0.gtx  175 65
Report:
  Location: (180P,420L)
  Band 1:
    Value: 4.43069982528687