Adding an option for not filtering zeros at start of reported case time series

[LloydChapman]

For my particular use case, it would be helpful to have an option not to filter zeros in the reported case time series before the first reported case as is currently done in create_clean_reported_cases, i.e. to add an option to epinow and estimate_infections to switch the filtering on and off. I'll create a pull request with this feature.