Added new command line option to channelmod: noise_power_dBFS. This options
allows the user to configure noise power in dBFS added instead of the per-channel
value.

This makes it so noise is not accumulated per channel reaching higher than expected
values.

The dBFS unit allows easy configuration. The gNB by default uses -36 dBFS setting.
To reach 3dB SNR one should set the noise_power_dBFS value to -39.

To configure using command line use --channelmod.noise_power_dBFS -50. Using this
setting the gNB and nrUE connect without issue.