[h5] DataArray: fix removal of polynomCoefficients

polynom_coefficients is not an attribute but a dataset in
hdf5, so removing it must remove the dataset not an
attribute. Add a missing test for it.
DataArray::getData: add support expansionOrigin
[test] checks for expansionOrigin (+ polynomial)
Merge pull request #379 from gicmo/expansionOrigin

DataArray::getData: Add support for expansionOrigin