TURTLE v0.4

This new release cleans and simplifies the TURTLE API. Its main features are:

• Unify maps and tiles as a single object
• Migrate ECEF routines to a separate name space and rename the turtle_datum object to turtle_stack.
• Add support for geoids as tabulated maps (add GRD format for EGM96).
• Replace the turtle_box object with a turtle_map_info structure, gathering all map meta data (except any projection).
• Clean the error stream and cross module functions.
• Provide a default error handler for library errors, with a hard exit.
• Package light dependencies with the source, i.e. JSMN and TinyDir.

Note that several API functions and objects have changed with this update.