simplify single axis single figure

[florisvb]

In cases where the user wants to have each axis get output as one figure (layer in inkscape), simplify the processing to reduce the number of figurefirst tags necessary to make it work.

Ideas:

(1) Add a figurefirst python option to create a new figure for axes that are not associated with figures (2) Add a figurefirst xml tag to figurefirst:axis to specify the creation of a new figure for that axis