Draft: WIP: Proposal for power control for rfsimulator

This implements rx_gain, tx_gain and tx_power for rfsim. A constant sample reference amplitude value at 0 dBm is selected to be 256. When calculating power, power == 10log10(rms^2) - 10log10(rfsim_k_constant) rfsim_k_constant = rms^2 at 0dBm == 256*256*2.

This is not integrated with any softmodem so far but some tests are added to show how power could be calculated with rfsim.