DataArray: Add createData function and implement it for HDF5

Add this createData() function to:
- IDataArray
- DataArrayHDF5
- DataArray
DataArray: Add createData<T> for convenience
[test] DataArray::testData init the test array with 42.0 not 0.0

So we can actually check if the writing worked. Doh!
[test] DataArray::testData add checks for createData
Merge pull request #104 from gicmo/dataarray_createData

New methods on DataArray to explicitly create the underlaying data, i.e. the DataSet