Simple Arrow: Fix d.head argument

[discoleo]

Simple Arrow: arrowSimpleH Function

The d.head argument is not used inside the code:

• fix: dV = d.head;

  ### Head
  ahead  = list(arrHS(x[2], y[2], slope=slope, d=d, dV = d.head), lwd = h.lwd);

The default arguments can be also improved:

• especially d = -0.5 and d.head = c(-d, d);

  arrowSimpleH = function(x, y, lwd=1, h.lwd=lwd, col="red",
      d = -0.5, d.head=c(-d, d), d.lines=0) {

[DarianFlorianVoda]

Changed