When annotated colors are provided, coerce to character vector

[jiajic]

When an annotated colors in GiottoVisuals are provided, providing as list can result in a very unclear error message. For example:

cell_colors <- list(
    "T_Cell" = "darkgreen",
    "Macrophage" = "cornflowerblue",
    "Endothelial" = "red",
    "Adipocyte" = "purple",
    "B_Cell" = "blue",
    "Mast" = "green",
    "PVL" = "lightblue",
    "CAF" = "orange",
    "Basal_like_Epithelial" = "brown",
    "Cancer" = "grey"
)

plotUMAP(gobject, cell_color = "cell_types", point_size = 0.2,
         cell_color_code = cell_colors,
         point_border_stroke = 0,
         label_size = 4,
         save_param = list(
             base_width = 11
         ))

This will eventually produce an error and ALSO lock up your session for quite a long time as ggplot figures things out.

These cases should be coerced to the expected color vector if possible so that it can be gracefully handled.