External Cells

- Class ExtCell replaced with an OMNeT++ Module.
- An ExtCell produces power on the available bands according to a configurable pattern.
- computeExtCellInterference() function in the Realistic Channel Model was modified accordingly.
- Three interference model added:
    a) the cell allocates all the bands
    b) the cell allocates X bands, which are chosen randomly (X is a tunable parameter);
    c) the cell allocates X contiguous bands, starting from a given band Y (X, Y are tunable parameters).