add support for units to DataTag
disallow "" as valid unit, must be given as "none"
moved docstring of unit sanitizer to util header
none unit testing, fixes
improve getOffsetAndCount to handle optional extents and units
add test case for valid and invalid units in DataTag
enable unit sanitizer during setting of units, added test
[test] Fix all places where test for definition() was wrong

This breaks a lot of tests:

68% tests passed, 7 tests failed out of 22
The following tests FAILED:
	  3 - DataArray (Failed)
	  5 - DataTag (Failed)
	  7 - Entity (Failed)
	 16 - Property (Failed)
	 17 - Section (Failed)
	 18 - SimpleTag (Failed)
	 19 - Source (Failed
NamedEntityHDF5::definition() only set optional if getAttr == T

Fixes the NamedEntityHDF5::definition part of issue #233
Merge pull request #242 from gicmo/issue_233_definition

Fix NamedEntity::definition to correctly return none
this time in the right order :-)
unit setter improved using transform and lambda construct
Merge pull request #241 from jgrewe/data_tag_unit

Add unit support for DataTags ...

LGTM ;-)