create assessment connecting epiparameter with cfr and epichains

[avallecam]

create assessments with delay distributions not present in epiparameter but using the epidist interface as a more efficient input for cfr and epichains

this is in contrast to epinow2, which has its internal distribution interface

ideas in

• Add how to use a conversion function to build a custom epidist class object epiparameter#114 (comment)
• can we use as.function() to convert object to the generate() instead of density()? simulist#107 (comment)
• how to pass multiple <epidist> objects to one (or multiple) {cfr} data inputs howto#36