Coordinates should accept single categorical values

[bryevdv]

Categorical coordinates can only easily be passed in CDS columns. Trying to pass a single values results in this:

# nonexistent column name error
p.circle("foo", "bar", line_width=7, size=50, **options)

# property validation error (expects a number)
p.circle(value("foo"), value("bar"), line_width=7, size=50, **options)

# works but clunky
p.circle(["foo"], ["bar"], line_width=7, size=50, **options)