Don't try to unbox lists

[keller-mark]

Describe the bug

In https://github.com/vitessce/vitessceR/blob/main/R/config.R#L210, we only should be unboxing scalar values, not lists/vectors.

To Reproduce

Try to set cellSetColor coordination value,

vc$link_views(
c(scatterplot_pca, scatterplot_tsne),
c(CoordinationType$CELL_SET_COLOR),
c_values = list(list(obj_list(path = c("Cell Type Annotations", "Astrocyte", "Astrocyte Gfap"),
color = as.vector(col2rgb("blue")))))
)
in json_remote.Rmd vignette

Throws Error: only atomic vectors of length 1 or data frames with 1 row can be unboxed.

Expected behavior
No error.