Integrate infection duration into InfectiousnessRateFn. Proposal to close #55.

[ChiragKumar9]

• Although infection duration is still specified as a global parameter, this structure enables each rate function type to have their own definition of infection duration and for each rate function instance to have a different value of the infection duration.
• If we switch to the structure in Specify rate functions in input.json #62, infection duration can be specified for each rate function at input.
• This PR does not solve the problem of needing to model symptom recovery, and a subsequent PR will introduce a ClinicalDiseaseSymptomStatus and tracker.