Rename <epidist> with <epiparameter>

[joshwlambert]

Breaking changes

This PR addresses #325 by renaming the main class in {epiparameter} from <epidist> to <epiparameter>. This is due to potential conflict or confusion with the {epidist} R package.

Function names that previously used epidist now use epiparameter (for exceptions see below). These include: epiparameter_db(), epiparameter(), as_epiparameter(), etc. The other class used in {epiparameter}, <multi_epidist> has also been renamed to <multi_epiparameter>.

Naming the class after the package also follows the idiomatic convention of other R packages, e.g. {incidence} and <incidence>, {epicontacts} and <epicontacts>.

The other change made in this PR is to shorten the names of the create_*() and .clean_*() functions by removing epidist from the name. If there is a conflict with other functions with the same name from another package then the functions can be explicitly namespaced (epiparameter::create_uncertainty()).

[jamesmbaazam]

This renaming exercise looks good to me. My only concern is that this package might already have a good number of users who will be affected by this change without prior warning.

A better approach would be to gradually deprecate the functions. I'm sure you have good reasons for going this route but I just thought to bring it up for discussion.

[joshwlambert]

@jamesmbaazam I appreciate this being raised. In general for breaking changes on packages that are < v1.0.0 and are not on CRAN I've made breaking changes without a gradual deprecation process. I always record any breaking changes in the NEWS.md file.