Implement multiband likelihood

- bilby_pipe/parser.py: add parameters for multiband likelihood
- bilby_pipe/input.py: add waveform generator and likelihood for the multiband technique
- bilby_pipe/data_generation.py: add method to save multiband weights
- bilby_pipe/data_analysis.py: load dumped weights
- bilby_pipe/plot.py: changes for multiband source model
- examples/other_examples/bbh_multiband.ini: add example
- tests/input_test.py: add unit test for multibanding