Update frequency table variable names and logging statements.

[tadscottsmith]

Resubmitting against the master branch.

This helps clean up some of the P25 frequency table (Bandplan) information variable names and logging statements.

1. Renames Channels to Freq_Tables to align with OP25. https://github.com/boatbod/op25/blob/883c569c040cdfade78fda245929e2e9eba6df2a/op25/gr-op25_repeater/apps/tk_p25.py#L355-L373
2. Renames channel_id_to_string function to channel_id_to_freq_string to help clarify what is being returned.
3. Adds a channel_to_string function that prints out the channels in the format of <XX>-<YYYY> where XX is the bandplan ID and YYYY is the channel ID.

Following this PR, I plan to add a second PR to expand and add the ability to input custom frequency tables for systems that aren't correctly sending them over the control channel.

[robotastic]

Make it so! 🖖