Set Rhd2164 DspCutoff to Off

[bparks13]

To give the widest pass band possible, the DSP cutoff value is now set to Off.

Fixes #543

[aacuevas]

I must say, I'm not 100% convinced defaulting the DSP to disabled is the best option
Just as a reminder of what de DSP does: The amplifiers on the Intan chip, past the analog filter, introduce a DC offset that varies with channel. The DSP exists to remove this DC offset and ensuring that all signals are 0-centered. With it disabled, all the signals will appear centered about different values.

Most analysis do their own filtering afterwards, so it is true that in most cases this just adds an unnecessary additional FIR filtering (with all its potential effects). But it will be very noticeable when plotting the signals.

So if we set this to off by default, we need to thoroughly document this.

[jonnew]

I understand but having it off is in line with the the least filtering of data by default principle that we tend to follow. e.g. the analog settings are both maxed out.

What does to OE GUI acq. board plugin do by default?

In any case, the text in your comment can simply go in the XML description of this setting!

[aacuevas]

The GUI defaults to DSP enabled at 0.5Hz and analog filters set at 1.0Hz to 7.5KHz.

I'm ok with this library have defaults that are just the least filtering, I just thought this particular setting needs to be specially clear, since it is not clear at the get-go why is it needed in the first place, or why the data has a DC offset without it.

We might want to throw a line somewhere in the docs about that. But for sure, having this in the xml comments I believe is a good idea and we should!