fix theater label in viewer for contextless plots
fix theater label in viewer for contextless plots

If a plot `'figname'` is logged outside of any `log.context`, the theater
showed `'undefined/figname'`.  This patch checks if a plot is contained in a
context and if not omits the (undefined) context name.

joostvanzwieten/nutils,wijnandhoitinga/nutils,nutils/nutils,CVerhoosel/nutils