Add support for OMERO_SESSION_DIR environement variable

With this commit, having OMERO_SESSION_DIR set should allow all CLI commands
to default to another sessions directory than the default one.
Suppress the session-dir argument help and expose the envvar
Fix extended help to use OMERO_SESSIONS_DIR
Add unit tests for sessions store directory
Fix OMERO_SESSION_DIR help indentation
Fix narginchk in getTile()
Update develop submodules

Repository: openmicroscopy/openmicroscopy
Already up-to-date.

Repository: openmicroscopy/bioformats
Updating 0819d22..edaee60
Previously merged:
  # PR 1331 rleigh-dundee 'C++: Add FormatWriter interface'
  # PR 1335 melissalinkert 'ND2: prevent NumberFormatExceptions when parsing text data (rebased onto develop)'
  # PR 1324 melissalinkert 'Deltavision: conditionally ignore stage position values of 0'
  # PR 1317 melissalinkert 'Gatan: fix physical dimension parsing when the Z size is missing'

Repository: ome/scripts
Already up-to-date.

Repository: openmicroscopy/ome-documentation
Updating 01a91b2..9f72455
Previously merged:
  # PR 987 snoopycrimecop 'Update auto-generated properties'
  # PR 984 aleksandra-tarkowska 'adding support for django directives'

Generated by OMERO-5.1-latest-submods#196 (http://ci.openmicroscopy.org/job/OMERO-5.1-latest-submods/196/)
Fix malformed reference
Merge pull request #3038 from ximenesuk/enable-fs-tests

Re-enable fs tests
Merge pull request #3025 from will-moore/preview_panel_2

Preview panel 2
Merge pull request #2962 from sbesson/omero_src_cli_flake8

Flake8 omero.cli
Merge remote-tracking branch 'origin/develop' into omero_session_dir

Conflicts:
	components/tools/OmeroPy/src/omero/cli.py
flake8: mark import-only file all as noqa
flake8: mark import-only file api/init as noqa
flake8: mark import-only file min as noqa
flake8: fix java.py
flake8: fix tables.py
Fix flake8 warnings
flake8: update config in include fixed files
Do not cast channel start/end to int.