#127 Allow inputs to have different rates while concat'ing
There is a restriction that when using any combiner mode except "sequence", all the input files must be the same rates.
This makes sense for mix and merge combiners but is questionable for concat mode. The main reason we do not support it (I think) is because originally it was difficult to create a new effects chain (with new rate effect options) as we transitioned from one input file to the next and while still re-using a pre-existing output file "effect". The sequence combiner resolved it by creating a new output "effect" at each transition.
We now have logic that allows creating a new effects chain while reusing the old output effect; which was needed by the ":" multiple effects chain feature. So now we can easily support each input having a different rate in concat mode and write to the same output file/device.
We should add support for concat combiner having inputs of different rates. Please note that I think we should continue to support sequence combiner even with this change. For sound cards that support resampling in HW, I think its more efficient to reopen the audio device for each input file then always resampling to the rate of the first file.
Alternatively, some audio drivers have bugs that cause "pops" each time the audio device is opened. Resampling each input file is a better option to avoid the pops.
