[test] RangeDimension::unit: fix test for none return case
[h5] RangeDimension::unit: return none if attribute is not set
[test] RangeDimension: test for none return of label property
[h5] RangeDimension::label: fix returning none if it is not there
Merge pull request #244 from gicmo/issue_233_rd

Fix none return for RangeDimension::label [h5]