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.