Support constructing stratigraphs from tidy data frames

[joeroe]

I.e.,

data.frame(
    unit = c("A", "B", "C", "C"),
    above = c(NA, "A", "A", "B")
)

Should have the same result as

tibble(
    unit = c("A", "B", "C"),
    above = list(NA, "A", c("A", "B"))
)

does now.

Don't know how common this format will be in the wild, but it's much easier for quickly constructing graphs in R itself.

Should be configurable, but also probably the default if label is not unique?