Subsetting multiple entries in as_epidist

[CarmenTamayo]

Currently, in order to create epidist objects from {epireview}, it's necessary to first manually subset their parameter database, to select a single row, and then use the as_epidist function.

Ideally, the process should be similar to how epidist_db works, where it's possible to create an epidist object by directly subsetting entries from the {epiparameter} database, and have either a single object if single_epidist = TRUE, or as many entries as those available in the database that meet the conditions specified by the arguments.
I think both options, i.e., creating an epidist from a single entry, as well as by filtering the {epireview} database, should be available to the user.

Further, for the most seamless user experience, the arguments included in as_epidist should be as similar as possible to those in epidist_db, i.e., disease, pathogen, epi_dist, distribution, author, subset and single_epidist.