[io] fix source output padding sizes
[io] source_pad_do_output to pad extra channels
[io] use memset in padding helpers
[io] add helpers to pad source output
[io] add helpers to check source output sizes
[io] add helpers to validate input
src/aubio_priv.h: move include config.h here
src/io/ioutils.h: add functions to check samplerate and channels, use in sink_*.c