Add new osmium::util::double2string() function and use it.

Both Location and Coordinates classes had functions to convert doubles to
strings, doing nearly, but not quite, the same.
Use lower case for const variable, it is not a MACRO.
Use new double2string() function.
Make coordinate precision for WKT and GeoJSON configurable.

The needed precision depends on the coordinates used. For OSM coordinates in
WGS84 7 digits after the decimal point is right. For Mercator 2 digits is
enough.