WIP PROJ6 support (#118)

PROJ6 support

- Convert lowercase epsg to uppercase EPSG
- Axis order of a CRS is no longer always easting, northing, it can also be northing, easting, e.g. EPSG:4326. If source and target CRS are available, axis order is adjusted using proj_normalize_for_visualization() to easting, northing if needed.
- Previously, GRASS did conversions of degress to/from radians and meters from/to map units. Now PROJ6+ might do these conversions itself, or not. GRASS is now doing the conversions only if needed.
- In PROJ6+, there can be several different operations to transform coordinates from one CRS to another CRS. If more than one operation is available, information is provided about these different operations. The current region is used by PROJ to select an appropriate operation.