code: function( v, k, m, s ){
  
              var x = v - m;
  
              // Mk = Mk-1+ (xk – Mk-1)/k
              // Sk = Sk-1 + (xk – Mk-1)*(xk – Mk).
              m += x / k;
              s += x * (v - m);
              return [m, s];
          }
docs: Physics.statistics.pushRunningAvg( v, k, m, s ) -> Array
  - v (Number): is value to push
  - k (Number): is num elements
  - m (Number): is current mean
  - s (Number): is current s value
  + (Array): Returns a 2 element array containing the next mean, and s value
  
  Push a value to a running average calculation.
  see [http://www.johndcook.com/blog/standard_deviation]
  
  Note: variance can be calculated from the "s" value by multiplying it by `1/(k-1)`