add mapping function with hard mapping to Modem class

[noc0lour]

I have copied some lines from the demodulation function to implement a mapping function. It should be possible to replace the original lines with a call to the new function if you don't want to have duplicates lying around.

[coveralls]

Coverage decreased (-0.1%) to 54.154% when pulling 364ccffeb3e4de9d0ecf8d766025b1b26e51cfd1 on noc0lour:add_mapping_function into 57f5ebd2498788dae71d4a213cadb3e59f45c11c on veeresht:master.