DataSet: Add dataType() function to return the DataType

Function is based on the hdf5::data_type_from_h5() function.
[test] DataSet: Test new dataType() method
IDataArray: Add new getDataType() method and HDF5 impl.
DataArray: Add getDataType() method

Directly wired to IDataArray::getDataType()
[test] DataArray: Small check to test getDataArray()
[test] DataSet::DataType use size_t in loop to silence warning
Merge pull request #108 from gicmo/DA_getDataType

DataArray: Add method to obtain the underlying DataType

OK