new InvalidUnitException
Property::unit checks for SI units, add testProperty::testUnit
moved unit checks and validation to Property.hpp
enabled unit checking in DataArray
implement SimpleTag = and << operators
fixed wrongly thrown runtime exception in SimpleTagHDF5.addReference()
Added TestSimpleTag. Fixed deleter bugs in units and extents, units
are ensured to be SI units.
unit checking enabled in Dimensions, unit test stub
fixed bug in RangeDimension copy assignment operator
unit checking in Dimensions enabled (restricted to SI units)

this restriction must go at some point. for now only "easily" scalable
units are supported.
new TestDimensions
restricted unit test in SimpleTag to atomic SI units.
Rename InvalidUnitException to InvalidUnit
Merge pull request #142 from jgrewe/unit_checking

Unit checking validation and SimpleTag tests

SimpeTag tests were added (part of issue #25)
Unit checking is issue  #95