reportlab_3_4_bugfix

[will-moore]

Fixes https://trello.com/c/KEIu1LhB/17-bug-reportlab-34-export

This fixes an error seen when exporting a figure with lots of images, so that the Info section is more than 1 page. Only seen with reportlab 3.4.0.

To test:

• need to install reportlab 3.4.0
• Add lots (e.g. 20) images to a figure and export PDF
• Check that you can export 2 or more info pages
• NB: Also good to check that this doesn't break using reportlab installed via yum install python-reportlab as on web-dev-merge.

[jburel]

Note that the web-* do not have that version of reportlab installed.

[jburel]

eel (python-reportlab):

• Export a figure with 45 images: one single page, 3 pages of "info" are added. Export works as expected
• Export a figure with 45 images: 3 pages, 3 pages of "info" are added. Export works as expected

[jburel]

While reviewing this PR, open with on when a dataset is selected will be a nice feature to have

[jburel]

Repeated the same tests with reportlab 3.4.0
everything works as expected
Thanks @will-moore
merging so it is included in the coming release