Add transformation accuracy

[kbevers]

We should be able to display the accuracy of a transformation between to CRS's. This would probably be best placed in a metadata end-point of /trans/, e.g /v1.1/trans/<from>/<to>/metadata which should return something like:

{
	"crs_src": "EPSG:4258",
	"crs_dst": "EPSG:4326",
	"accuracy": "1.0 m",
	"valid_bbox": [32.88,-16.1,84.17,40.18],
	"area_of_use": "Europe - ETRS89",
	"usage": "ETRS89 and WGS 84 are realizations of ITRS coincident to within 1 metre. This transformation has an accuracy equal to the coincidence figure.",
	"proj_string": "+proj=noop"
}

For EPSG registered CRS's this is easy to provide. Somewhat more complicated for DK:S34S etc but where there's a will there's a way.