#238 psradd : check frequency and patch missing channels

[Gregory Desvignes]

When using the command "psradd -R -o sum.ar <files> " to append
different files produced by different computers (at
different but adjacent frequencies !), psradd seems to not use the
frequency in the header when one of these files is missing. It's just
stacking files without checking the frequency.
psradd could instead patch the missing channels between two different files at two different frequencies