docs: public double errorMembership( double[] sampleA ) {
          double[] eig = sampleToEigenSpace(sampleA);
          double[] reproj = eigenToSampleSpace(eig);
  
  
          double total = 0;
          for( int i = 0; i < reproj.length; i++ ) {
              double d = sampleA[i] - reproj[i];
              total += d*d;
          }
  
          return Math.sqrt(total);
      }