GH-89: initial support for NTv2

[barthanssens]

Initial NTv2 parser (only one subgrid, little endian format), as suggested in #89

Signed-off-by:Bart Hanssens bart.hanssens@bosa.fgov.be

[pomadchin]

Wow, this is so nice! Could you add some test coverage as well?

[barthanssens]

Sure, still tinkering with it (I guess I could add detection of endianness).

Currently it seems to work for a gsb file used in Belgium (https://www.ngi.be/website/wp-content/uploads/2020/07/NTv2-1.zip, CC-BY, about 5 MB size), but open to suggestions for smaller/other test files.

[pomadchin]

@barthanssens I was able to find europe/100800401.gsb 25.2 KB

Hopefully should work?

### Spain (Catalonia): ED50 -> ETRS89

*Source*: [Institut Cartogràfic i Geològic de Catalunya (ICGC)](http://www.icgc.cat/content/download/48766/337962/version/1/file/Malla_NTv2_11_03.zip)  
*Format*: NTv2  
*License*: [Creative Commons Attribution 4.0](http://creativecommons.org/licenses/by/4.0/)  
*Source CRS*: EPSG:4230 (ED50)  
*Target CRS*: EPSG:4258 (ETRS89)  
*Used by*: [EPSG:5661 Transformation](https://www.epsg-registry.org/export.htm?gml=urn:ogc:def:coordinateOperation:EPSG::5661)

* 100800401.gsb

Source

[barthanssens]

Thanks @pomadchin , I'll give that one a try

[pomadchin]

🎉 Thank you and congratz with the first contribution!