Refs #6541. First cut at axis labels.

This first pass gets things working for the NeXus reader.
Refs #6541. Axis labels for MDWorkspace sources.
Refs #6541. Adding axis labels for the rebinners.
Refs #6541. Adding some tests.
Refs #6541. Enable axis labels for SQWEvent reader.
refs #6598 added test and descriptions.
(cherry picked from commit 24cc4e60f36d63ea9452955f6bc2199a2ee87981)
refs #6598 Take out boost optional as quick fix.
(cherry picked from commit 2507b7f4cda3f915e41ecb4d6023fcf6f67f2a3c)
Refs #6541. Checking axis titles.

Comparing the axis titles in the resulting datasets for the MDEW and MDHW
loading presenters.
Refs #6541. Fixing test to use correct workspace type.
Refs #6541. Add XML helper objects for MDEventTestHelper data.
Refs #6541. Adding rebinning presenter tests.
Refs #6541. Removing comments from tests.
Refs #6541. Fix SQW loading presenter to set axis labels.
Refs #6541. Adding SQW loading presenter test.
Refs #6541. Adding end points to SQW loading presenter test.
Refs #6541. Adding test for MDEWEventNexus loading presenter.
Merge branch '6541_3d_axis_label' into 6315_build_pv_398