code: static BIG modAdd(BIG a, BIG b, BIG m) {
          BIG c = a.plus(b);
          c.mod(m);
          return c;
      }
docs: Takes input BIGs a, b, m and returns a+b modulo m
  
  @param a the first BIG to add
  @param b the second BIG to add
  @param m the modulus
  @return Returns a+b (mod m)