Expose `power2mW` and `mW2power`

Hamlib (even back to [3.3](https://github.com/Hamlib/Hamlib/blob/301ebb92eaa538dfa75c06821f46715f40dd7673/include/hamlib/rig.h#L1380-L1389)) has rig methods `power2mW` and `mW2power` to translate between transmit power percent and actual milliwatts. This would be useful to expose in goHamlib.