Returns the values associated with @key under @group_name as
doubles.
If @key cannot be found then %NULL is returned and @error is set to
#G_KEY_FILE_ERROR_KEY_NOT_FOUND. Likewise, if the values associated
with @key cannot be interpreted as doubles then %NULL is returned
and @error is set to #G_KEY_FILE_ERROR_INVALID_VALUE.
Returns the values associated with @key under @group_name as doubles.
If @key cannot be found then %NULL is returned and @error is set to #G_KEY_FILE_ERROR_KEY_NOT_FOUND. Likewise, if the values associated with @key cannot be interpreted as doubles then %NULL is returned and @error is set to #G_KEY_FILE_ERROR_INVALID_VALUE.