Rename transmissibility to transmission rate

[pratikunterwegs]

This issue requests the renaming of the model parameter 'transmissibility' to 'transmission rate', in all function arguments and documentation.

Strictly speaking, $\beta$ is the effective contact rate ($R$ could also be called a 'transmssibility parameter'). Perhaps clearer to call $\beta$ the transmission rate (as long as consistent across the package).

Originally posted by @adamkucharski in #176 (comment)