Export figure no script

[will-moore]

This separates the figure export functionality from the OMERO.script so that we can export figures without running any scripts.

TODO: Need to decide how to distribute the export module. The script needs this on the server, and it could also be useful independently of the web app so it should probably be it's own pip installable app. Does this have to go in it's own separate repo?

Until then, the export script will not work:

--exclude

The export returns a file_object, without writing any images or thumbnails to disk (the export script previously wrote files to current folder since it was a temp script folder).

Example usage to save the PDF / TIFF or ZIP to disk:

from omero_figure.export import FigureExport

fig_export = FigureExport(conn, script_params)
file_object = fig_export.build_figure()
file_data = file_object.getvalue()
file_object.close()
file_name = fig_export.get_export_file_name()
with open(file_name,'wb') as out:
        out.write(file_data)

For export to new Image in OMERO (or multiple images for multi-page figure), we get a list of new Image IDs:

fig_export = OmeroExport(conn, script_params)
image_ids = fig_export.build_figure()

This makes many of the same changes as https://github.com/ome/omero-figure/pull/240 but we still use the Scripting Service to do the exports for the web app so the behaviour is unchanged.

cc @jburel @joshmoore @carandraug

[will-moore]

As expected, the export script is failing in travis integration tests, since the export module is not included on the server:

E         File "./script", line 26, in <module>
E           from omero_figure.export import FigureExport, \
E       ImportError: No module named omero_figure.export

[will-moore]

cc @jburel need to discuss the next steps here