Provides IO interface across multiple IO objects.
@param [Array<IO>] ios Array of IO objects
Reads and returns partial content acrosss multiple IO objects.
@param [Integer] length Number of bytes to retrieve @param [String] outbuf String to be replaced with retrieved data
@return [String, nil]
Rewinds all IO objects and set cursor to the first IO object.
Returns sum of all IO sizes.
[Validate]