Introduce SRS physical simulator and improvements

This MR consists of two parts:

- Introduces SRS physical simulator to test different configurations of
  SRS
- Improvements in the SRS channel estimation (Addressing the comments
  from old, unfinished MR !2875)
  - rewrite nr_srs_channel_estimation() to loop over rx antennas and
    antenna ports to be able to parallelize
  - Improvements in timing_advance_offset and timing_advance_offset_nsec
    computation
  - Introduce SRS multi-symbol channel estimation
    - The channel impulse response is estimated from the averaged
      channel frequency response over multiple symbols
    - The channel frequency estimates are left as they are to be able to
      estimate doppler in the future
  - Simplify SRS channel estimation to handle memory alignment in the
    case of different SRS configurations (eg: comboffset)

To use multiple symbols when running gNB, the following parameters needs
to be edited in the function get_srs_resource()

- srs_res->resourceMapping.startPosition with a range from {0,1,..,5}
  corresponds to the start symbol {13, 12, .. 8} with in a slot
- srs_res->resourceMapping.nrofSymbols indicates the number of symbols
  {n1, n2, and n4}

Make sure the that start symbol + number of symbols is < 14

The timing results for SRS in develop and srs_phy_simulator are as follows:

Command used: sudo ./nr_ulsim -n1000 -s50 -S50 -R273 -W2 -y2 -z2 -E 1 -P

develop

|__ RX SRS time                            124.94 us (1000 trials)		(124.94 total [ms])
    |__ Generate SRS sequence time           0.05 us (1000 trials)		(  0.05 total [ms])
    |__ Get SRS signal time                 19.41 us (1000 trials)		( 19.41 total [ms])
    |__ SRS channel estimation time         89.11 us (1000 trials)		( 89.11 total [ms])
    |__ SRS timing advance estimation time  13.10 us (1000 trials)		( 13.10 total [ms])
    |__ SRS report TLV build time            2.80 us (1000 trials)		(  2.80 total [ms])
        |__ SRS beam report build time       0.00 us (  0 trials)
        |__ SRS IQ matrix build time         2.73 us (1000 trials)

srs_phy_simulator

|__ RX SRS time                            211.93 us (1000 trials)		(211.93 total [ms])
    |__ Generate SRS sequence time           0.04 us (1000 trials)		(  0.04 total [ms])
    |__ Get SRS signal time                 19.55 us (1000 trials)		( 19.55 total [ms])
    |__ SRS channel estimation time        152.05 us (1000 trials)		(152.05 total [ms])
    |__ SRS timing advance estimation time  36.67 us (1000 trials)		( 36.67 total [ms])
    |__ SRS report TLV build time            2.96 us (1000 trials)		(  2.96 total [ms])
        |__ SRS beam report build time       0.00 us (  0 trials)
        |__ SRS IQ matrix build time         2.88 us (1000 trials)

Note that in srs_phy_simulator, to compute timing offset ns, an
oversampling factor of 2 is performed. Also, for peak detection, we
average the channel estimates for every port.