[test] DataArray: Fix test for unit property (none condition)

This test will currently fail due to issue #233
[test] DataArray: Test for expansionOrigin (none condition)

This test will currently fail due to issues #233, #238, #239
DataArrayHDF5::unit: Fix getter for unit() [none case]

Should fix the DataArray::unit part of issue #233
DataArray::expansionOrigin: use correct backend function

Closes issue #238
DataArrayHDF5::expansionOrigin(none) use correct attribute

Use 'expansion_origin' not 'expansionOrigin'. Closes issue #239
DataArrayHDF5::expansionOrigin() fix none case in getter

This fixes DataArrayHDF5::expansionOrigin part of issue #233
Merge pull request #240 from gicmo/issue_233_238_239

DataArray optionals (expansionOrigin, unit) none case fixes