[test] Dimension: SampledDimension::label: fix test for none
DimensionHDF5::label: correctly return none if attr is not set
[test] SampledDimension::unit: correctly test for none return
DimensionHDF5::unit: correctly return none if attr is not set
Merge pull request #243 from gicmo/issue_233_sd

Fix none returns for SampledDimension