Add offset to DB and Marziano et al. Mpox parameters

[joshwlambert]

This PR adds the offset parameter to all database entries. For existing entries a default offset of zero is specified (i.e. no offset).

The Mpox incubation period, serial interval and generation time parameters from Marziano et al. (2024) are also added to the database. These entries contain a non-zero offset. There are multiple entries for each incubation period, serial interval and generation time for estimates reported using different datasets or using non-censored and double-interval censored fitting methods.

The data dictionary is updated to document the offset parameter and to require it for each database entry.

Documentation and tests are updated where required.