Added bounds check in CalcProb

    - Previously there was no bounds check on the potential
      hop sites. This will have lead to out of bounds requests
      for grid sites that did not exist.

    - Also changed sites on perifery to have an electric potential
      of zero rather than one.

    - Previously sites added to the discharge were not correctly
      set to Phi[site] = 1.0. This seems to have improved the
      behaviour with the discharge pattern branching out more rather
      than staying in the centre.
Runs fine but gives Aborted (core dumped)

    - Will have to look into this with Valgrind to try and
      figure out what's causing this. Other than that it
      looks to be running correctly.