Add AccessPoint class from which CellBase inherits

Add the AccessPoint class from which CellBase class now inherits from.
Common code in CellBase was moved to the AccessPoint class.
Move more code from CellBase to AccessPoint class

Also change the cell_id_fontsize attribute to id_fontsize when moving it
to the AccessPoint class.
Change METIS sim to not use Cluster class anymore

Change the METIS simulator to not use Cluster class anymore to generate
the base stations. Now we manually create the rooms as Rectangle class
objects and manually place the base stations as AccessPoint
objects. This makes the simulator more flexible and allow us to
distribute transmitters as we want.
Change 'cell' to 'room' which is more more exact

Change 'cell' to 'room' which is more more exact with the true meaning.
Fix wrong user_relative_positions

Now the SINRs color grids are correct
Many refactorings and improvements to METIS sim

Many refactorings and improvements to METIS simulator to make ir more
flexible and to allow further changes to have different AP
distributions (not just one AP per room).
Merge branch 'AccessPoint'