PHY: make sf_extension a parameter

This parameter is meaningful in TDD, to decide
when to start DL at eNB side. Since there is a
need for the PA to be operational, we need to
transmit a bit before the DL subframe coming
after an UL subframe. (We transmit zeros.)

We used to use N_TA_offset which may be too much.
Default value is now N_TA_offset/2 and can be
changed in the configuration file, in the RUs
section, like:

RUs = (
    {
       local_rf       = "yes"
         nb_tx          = 1
         nb_rx          = 1
         att_tx         = 0
         att_rx         = 0;
         bands          = [7];
         max_pdschReferenceSignalPower = -27;
         max_rxgain                    = 105;
         eNB_instances  = [0];
         sf_extension = 312;
    }
);

N_TA_offset is 624 (for 30.72MHz). In the example
above, we set sf_extension = 312, which is also
the default.

The value to put in the configuration file is for
30.72MHz. The value is scaled accordingly at runtime
(thus only one value to set for every RB configuration,
25, 50 or 100, leading to less problems when adapting
configuration files).

This option is for experts and should not be changed
randomly.