asdf_set_extension_type function for extension types #123
Description
Following #104 it should also be possible to set a value from an extension type. Extensions by default would generate their own asdf_set_<type> function (e.g. asdf_set_ndarray), but it will also be useful to have some generic
asdf_value_err_t asdf_set_extension_type(asdf_file_t *file, const char *path, asdf_extension_t *ext, void *value);
mirroring the existing asdf_get_extension_type.
Generated functions like asdf_set_ndarray are just a wrapper around this.