Add offset to <epiparameter>, class methods and helper functions

[joshwlambert]

This PR adds the functionality for an <epiparameter> object to have a distribution offset. See epiverse-trace/epiparameterDB#24 for addition of offset parameters to the database.

An offset argument is added to create_prob_distribution(), with the offset being stored as a class attribute of the returned object. This avoids the requirement of the distribution object to handle offsets explicitly (for example {distributional} and {distcrete} currently do not handle offsets).

The <epiparameter> distribution functions (density(), cdf(), quantile() and generate()) are updated to use the offset. By default, a zero offset will produce the same behaviour as previous versions.

The <epiparameter> format() and print() methods are updated to print the offset if it is non-zero.

Two unit tests are added that check create_prob_distribution().

This PR also temporarily adds the dependency on {epiparameterDB} from the @offset branch. The dependency will be removed from a specific branch when the Mpox parameters are merged into main in {epiparameterDB}, and the remote dependency will be removed once the new version of {epiparameterDB} is on CRAN.

[github-actions]

This pull request:

• Adds 1 new dependencies (direct and indirect)
• Adds 1 new system dependencies
• Removes 1 existing dependencies (direct and indirect)
• Removes 1 existing system dependencies

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