Review release

• Add a vignette defining the package models.
• Updated documentation to use {preferably}.
• Add near coverage target tests
• Add a new utility, fv_example() which can be used to load built in example output and scripts. This is used for postprocessing and plotting examples as well as for testing plotting on static input.
• Model-specific functions (i.e load_model, summarise_posterior, and functions prefixed with stan_ have been renamed with the fv_ prefix).
• Added the ability to pass model-specific functions to the forecast function and wrappers enabling users to supply custom models but make use of the remaining package functionality.