Create a vpcdb object, and standardize parameter checking

Usage

as_vpcdb(..., type = NULL, facet = NULL, scales = NULL, labeller = NULL)

...: Extra parameters (not checked) added to the object
type: The type of vpc (e.g. "continuous", "categorical", "censored", or "time-to-event")
facet: either "wrap", "columns", or "rows"
scales: Are scales shared across all facets (the default, "fixed"), or do they vary across rows ("free_x"), columns ("free_y"), or both rows and columns ("free")?
labeller: ggplot2 labeller function to be passed to underlying ggplot object

A vpcdb object which is simply a named list with some of the values checked for correctness