cpp: Adapt to Boost 1.56 optional changes

- Explictly dereference optional values
- Use double negative for comparing to true
merge develop -Dorg -Ijburel/openmicroscopy:qa-submit-new -Ssuccess-only: PR 1296 (Add test for getRequiredDirectories)
merge develop -Dorg -Ijburel/openmicroscopy:qa-submit-new -Ssuccess-only: PR 1369 (Document usage of setSeries() and getIndex() in MATLAB documentation)
merge develop -Dorg -Ijburel/openmicroscopy:qa-submit-new -Ssuccess-only: PR 1370 (cpp: Adapt to Boost 1.56 optional changes)
merge develop -Dorg -Ijburel/openmicroscopy:qa-submit-new -Ssuccess-only

Repository: openmicroscopy/bioformats
Excluded PRs:
  # PR 1365 ctrueden 'PrairieView 5.2 fixes and improvements' (label: exclude)
  # PR 1309 melissalinkert 'Allow tile-based access of large JPEG-2000 images' (exclude comment)
  # PR 1362 melissalinkert 'Make FormatReader's RandomAccessInputStream transient' (label: breaking)
  # PR 1265 qidane 'New units' (label: breaking)
  # PR 857 melissalinkert 'Initial support for Zeiss .lms files' (label: exclude)
  # PR 1366 ctrueden 'Miscellaneous cleanup' (label: exclude)
  # PR 1015 qidane 'Messy binary' (exclude comment)
  # PR 1177 rleigh-dundee 'specification: Add metadata for HDF5 embedding' (exclude comment)
  # PR 1161 sommerc 'Initial CellH5 Reader class' (user: sommerc)
  # PR 858 melissalinkert 'Initial support for Veeco AFM data' (label: exclude)
Already up-to-date.

Merged PRs:
  # PR 1296 melissalinkert 'Add test for getRequiredDirectories'
  # PR 1369 sbesson 'Document usage of setSeries() and getIndex() in MATLAB documentation'
  # PR 1370 rleigh-dundee 'cpp: Adapt to Boost 1.56 optional changes'

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