Merge pull request #75 from busstoptaktik/gridshift

Gridshift bug fix and improvements:

- Repair grid interpolation. Now replicates PROJ results exactly.
- Add some tiny NTv2 files:
  - 5458.gsb: A NTv2 version of the existing test.datum file (54N..58N, 8E..16E),
  - 5458_with_subgrid.gsb: As 5458.gsb, but with a denser subgrid at (55N..56N, 12E..14E)

geodesy/gsb/5458.gsa

@@ -0,0 +1,71 @@
+NUM_OREC 11
+NUM_SREC 11
+NUM_FILE 1
+GS_TYPE  SECONDS
+VERSION  2.0
+SYSTEM_F INTER
+SYSTEM_T GRS80
+MAJOR_F  6378388.0
+MINOR_F  6356911.946127946
+MAJOR_T  6378137.0
+MINOR_T  6356752.314140356
+
+SUB_NAME 5458
+PARENT   NONE
+CREATED  20231110
+UPDATED  20231110
+S_LAT    194400.0
+N_LAT    208800.0
+E_LONG   -57600.0
+W_LONG   -28800.0
+LAT_INC  3600.0
+LONG_INC 3600.0
+GS_COUNT 45
+
+16 54 0 0
+15 54 0 0
+14 54 0 0
+13 54 0 0
+12 54 0 0
+11 54 0 0
+10 54 0 0
+09 54 0 0
+08 54 0 0
+16 55 0 0
+15 55 0 0
+14 55 0 0
+13 55 0 0
+12 55 0 0
+11 55 0 0
+10 55 0 0
+09 55 0 0
+08 55 0 0
+16 56 0 0
+15 56 0 0
+14 56 0 0
+13 56 0 0
+12 56 0 0
+11 56 0 0
+10 56 0 0
+09 56 0 0
+08 56 0 0
+16 57 0 0
+15 57 0 0
+14 57 0 0
+13 57 0 0
+12 57 0 0
+11 57 0 0
+10 57 0 0
+09 57 0 0
+08 57 0 0
+16 58 0 0
+15 58 0 0
+14 58 0 0
+13 58 0 0
+12 58 0 0
+11 58 0 0
+10 58 0 0
+09 58 0 0
+08 58 0 0
+
+END

geodesy/gsb/5458_with_subgrid.gsa

@@ -0,0 +1,99 @@
+NUM_OREC 11
+NUM_SREC 11
+NUM_FILE 2
+GS_TYPE  SECONDS
+VERSION  2.0
+SYSTEM_F INTER
+SYSTEM_T GRS80
+MAJOR_F  6378388.0
+MINOR_F  6356911.946127946
+MAJOR_T  6378137.0
+MINOR_T  6356752.314140356
+
+SUB_NAME 5458
+PARENT   NONE
+CREATED  20231110
+UPDATED  20231110
+S_LAT    194400.0
+N_LAT    208800.0
+E_LONG   -57600.0
+W_LONG   -28800.0
+LAT_INC  3600.0
+LONG_INC 3600.0
+GS_COUNT 45
+
+16 54 0 0
+15 54 0 0
+14 54 0 0
+13 54 0 0
+12 54 0 0
+11 54 0 0
+10 54 0 0
+09 54 0 0
+08 54 0 0
+16 55 0 0
+15 55 0 0
+14 55 0 0
+13 55 0 0
+12 55 0 0
+11 55 0 0
+10 55 0 0
+09 55 0 0
+08 55 0 0
+16 56 0 0
+15 56 0 0
+14 56 0 0
+13 56 0 0
+12 56 0 0
+11 56 0 0
+10 56 0 0
+09 56 0 0
+08 56 0 0
+16 57 0 0
+15 57 0 0
+14 57 0 0
+13 57 0 0
+12 57 0 0
+11 57 0 0
+10 57 0 0
+09 57 0 0
+08 57 0 0
+16 58 0 0
+15 58 0 0
+14 58 0 0
+13 58 0 0
+12 58 0 0
+11 58 0 0
+10 58 0 0
+09 58 0 0
+08 58 0 0
+
+SUB_NAME 5556
+PARENT   5458
+CREATED  20231110
+UPDATED  20231110
+S_LAT    198000.0
+N_LAT    201600.0
+E_LONG   -50400.0
+W_LONG   -43200.0
+LAT_INC  1800.0
+LONG_INC 1800.0
+GS_COUNT 15
+
+14.0 56.0 0 0
+13.5 56.0 0 0
+13.0 56.0 0 0
+12.5 56.0 0 0
+12.0 56.0 0 0
+14.0 55.5 0 0
+13.5 55.5 0 0
+13.0 55.5 0 0
+12.5 55.5 0 0
+12.0 55.5 0 0
+14.0 55.0 0 0
+13.5 55.0 0 0
+13.0 55.0 0 0
+12.5 55.0 0 0
+12.0 55.0 0 0
+
+END