Export big images as jpeg 10841 dev 4 4 (rebased onto develop)

[sbesson]

This is the same as gh-49 but rebased onto develop.

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Fix the last PEP8 issues noted in #45. Flake8 test should pass.

[sbesson]

--rebased-from #49

[sbesson]

NB: #49 has been included in develop as part of b00f440. But I realized the latter commit introduced an unwanted indent. This commit should fix this error and remove all drift between dev_4_4 and develop on this script

sebastien@jrs-macbookpro-25399:scripts ((10195f3...)) $ git diff origin/dev_4_4 sbesson/rebased/develop/export_Big_Images_as_jpeg_10841_dev_4_4 -- omero/export_scripts/Batch_Image_Export.py
sebastien@jrs-macbookpro-25399:scripts ((10195f3...)) $ 

[sbesson]

To test this PR, run the script on an image and then check the script log once the script finishes successfully.
The log should include Saving planes from image: xxx

[pwalczysko]

Works as expected. Ready to merge.