Adding ability to Notch 50Hz or 60Hz

Added code so that the user can enable a 50 Hz notch instead of a 60 Hz notch.  There is no GUI button for it.   Should we do one?  Once the user does it once, it's not like they'll want to toggle back-and-forth.  I mean, they're only going to have 60Hz interference or 50Hz.  Should we really clutter the GUI with this extra button?

Until we decide to add GUI button, a user desiring 50Hz has to go into the code (EEG_Processing.pde), find the `defineFilters()` function, and change `if (true)` to `if (false)`...

    //define filters...assumes sample rate of 250 Hz !!!!!
    private void defineFilters() {
    int n_filt = filtCoeff_bp.length;
    double[] b, a, b2, a2;
    String filt_txt, filt_txt2;
    String short_txt, short_txt2; 

    //loop over all of the pre-defined filter types
    for (int Ifilt=0;Ifilt<n_filt;Ifilt++) {

      //define notch filter that will be used along with all of the bandpass filters (except for the "no filtering" case)
      if (true) {
        //60 Hz notch filter, assumed fs = 250 Hz.  2nd Order Butterworth: b, a = signal.butter(2,[59.0 61.0]/(fs_Hz / 2.0), 'bandstop')
        b2 = new double[] { 9.650809863447347e-001, -2.424683201757643e-001, 1.945391494128786e+000, -2.424683201757643e-001, 9.650809863447347e-001 };
        a2 = new double[] { 1.000000000000000e+000, -2.467782611297853e-001, 1.944171784691352e+000, -2.381583792217435e-001, 9.313816821269039e-001  }; 
        filtCoeff_notch[Ifilt] =  new FilterConstants(b2, a2, "Notch 60Hz", "60Hz");
      } else {
        //50 Hz notch filter, assumed fs = 250 Hz.  2nd Order Butterworth: b, a = signal.butter(2,[49.0 11.0]/(fs_Hz / 2.0), 'bandstop')
        b2 = new double[] { 0.96508099, -1.19328255,  2.29902305, -1.19328255,  0.96508099 };
        a2 = new double[] { 1.0       , -1.21449348,  2.29780334, -1.17207163,  0.93138168 }; 
        filtCoeff_notch[Ifilt] =  new FilterConstants(b2, a2, "Notch 50Hz", "50Hz");
      }

     // ... etc, etc etc