DSP Cutoff Option

Dear all,
this is my first attempt to push to the openephys repository.
Hope I am doing things right.

As we discussed in issue #177 I think adding the option to enable/disable the DSP cutoff filter and select it's frequency would be a useful addition so I added such a button and textfield to the Rhythm Source.
I am saving also these settings in the xml file.

Best,
Nikolas