OSGB36 datum and +nadgrids=@null support

[bosborn]

• Renamed OSEB36 variable to OSGB36 (Ordnance Survey of Great Britain 1936)
• Changed OSGB36 Transform
  • from: 446.448, -125.157, 542.060, 0.1502, 0.2470, 0.8421, -20.4894
  • to: 446.448, -125.157, 542.06, 0.15, 0.247, 0.842, -20.489
• Added +nadgrids=@null support to grid parsing (based on proj4js)

The OSGB36 change prevents the need to change EPSG:27700 proj parameters

• from: +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +datum=OSGB36 +units=m +no_defs
• to: +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +towgs84=446.448,-125.157,542.06,0.15,0.247,0.842,-20.489 +units=m +no_defs

There are two transformation versions floating around, but I believe this change is to the correct values and passes the added tests.

• https://epsg.io/27700
• https://github.com/OSGeo/gdal/blob/dbcb76893c728d7b711d353e689b683a81a2b6d7/autotest/gdrivers/ecw.py#L712
• https://gis.stackexchange.com/questions/313761/why-does-r-set-crs-inconsistently
• https://github.com/ngageoint/projections-java/blob/18ca270259a05e753d373d9fcf943a66ca75ebc8/src/main/resources/projections.epsg.properties#L10

This still does not resolve issue #32 (see TODO in added tests in CoordinateTransformTest)