code: function( v, k, m, s ){
              var invK = 1/k
                  ,x = v.get(0) - m.get(0)
                  ,y = v.get(1) - m.get(1)
                  ;
  
              // Mk = Mk-1+ (xk – Mk-1)/k
              // Sk = Sk-1 + (xk – Mk-1)*(xk – Mk).
              m.add( x * invK, y * invK );
  
              if ( s ){
                  x *= v.get(0) - m.get(0);
                  y *= v.get(1) - m.get(1);
  
                  s.add( x, y );
              }
          }
docs: Physics.statistics.pushRunningVectorAvg( v, k, m[, s] )
  - v (Physics.vector): is vector to push
  - k (Number): is num elements
  - m (Physics.vector): is current mean
  - s (Physics.vector): is current s value
  
  Push a vector to a running vector average calculation.
  see [http://www.johndcook.com/blog/standard_deviation]
  
  Calculations are done in place. The `m` and `s` parameters are altered.
  
  Note: variance can be calculated from the "s" vector by multiplying it by `1/(k-1)`
  
  If s value is ommitted it won't be used.