Error when passing Path objects as output params

This is the current behavior.
RuntimeError: BandMath: error before execution, while setting parameter 'out' to '/tmp/test.tif': Expected a string for 'out' parameter in BandMath, got <class 'pathlib.PosixPath'> for /tmp/test.tif)

This is an easy fix, since we added check_input_param, we can do the same for output.
Or even better, we can replace check_input_param with check_io_param.