Returns the value associated with key under group_name as a
double. If group_name is NULL, the start_group is used.
If key cannot be found then 0.0 is returned and error is set to
G_KEY_FILE_ERROR_KEY_NOT_FOUND Likewise, if the value associated
with key cannot be interpreted as a double then 0.0 is returned
and error is set to G_KEY_FILE_ERROR_INVALID_VALUE
Returns the value associated with key under group_name as a double. If group_name is NULL, the start_group is used.
If key cannot be found then 0.0 is returned and error is set to G_KEY_FILE_ERROR_KEY_NOT_FOUND Likewise, if the value associated with key cannot be interpreted as a double then 0.0 is returned and error is set to G_KEY_FILE_ERROR_INVALID_VALUE