Code formatter: define and hook code formatting enforcement for github PRs

Code formatting differs across developers. We should be at least consistent since odd formatting makes reading code more difficult for obvious reasons.
The most prominent differences I am aware of are

• use spaces vs tabs
• differences in formatting "if statements" (as displayed here):

  if(XX){
  

  vs

  if (XX) {
  

• insonsitent code identations like displayed here:

      public void method() {
   int i = 0;
   i++;
        i +=2;
  

• odd identations of whole anonymous classes as displayed here

               class Listener {
  int i = 0;
   public void method()        {
               }
     }
  

• multiple empty lines for no obvious reason like shown here

  public class ZZ {
  
  
      int i = 0;
  
  
     i++
  }