Adding CBC background calculation function and background module

Adding a flexible function to calculate the CBC background, in a new module background.py. This is work in progress; original function by @adrian.macquet and wip with Adrian and @patrick-meyers .

TODO

• make sure function works with bilby injection set generation/add function to generate the injection dictionary (this should not be necessary, but a tutorial to use bilby_pipe_create_injection_file may be useful. Note https://lscsoft.docs.ligo.org/bilby_pipe/0.3.12/create_injections.html )
• unit tests