Selects the matching dataset and returns its path.
        
        Parameters
        ----------
        elangle : str
            Upper case ascii letter defining the elevation angle
        quantity : str
            Name of the quantity e.g. DBZH, VRAD, RHOHV...
            
        Returns
        -------
        dataset : str
            Path of the matching dataset or None if no dataset is found.
            
        Examples
        --------
        Get the hdf5 path of the DBZH dataset at lowest elevation angle
        
        >>> pvol = odimPVOL('pvol.h5')
        >>> dataset = pvol.select_dataset('A', 'DBZH')
        >>> print(dataset)
        '/dataset1/data1/data'

def select_dataset(self, elangle, quantity):
        """
        Selects the matching dataset and returns its path.
        
        Parameters
        ----------
        elangle : str
            Upper case ascii letter defining the elevation angle
        quantity : str
            Name of the quantity e.g. DBZH, VRAD, RHOHV...
            
        Returns
        -------
        dataset : str
            Path of the matching dataset or None if no dataset is found.
            
        Examples
        --------
        Get the hdf5 path of the DBZH dataset at lowest elevation angle
        
        >>> pvol = odimPVOL('pvol.h5')
        >>> dataset = pvol.select_dataset('A', 'DBZH')
        >>> print(dataset)
        '/dataset1/data1/data'

        """
        elangle_path = None
        try:
            search_results = self.search('elangle', self.elangles[elangle])
        except KeyError:
            return None

        if search_results == []:
            print('Elevation angle {} is not found from file'.format(elangle))
            print('File contains elevation angles:{}'.format(self.elangles))
        else:
            elangle_path = search_results[0]
                    
        if elangle_path is not None:
            dataset_root = re.search( '^/dataset[0-9]+/', elangle_path).group(0) 
            quantity_path = None
            search_results = self.search('quantity', quantity)

            for path in search_results:
                if dataset_root in path:
                    quantity_path = path
                    break
                    
            if quantity_path is not None:
                dataset_path = re.search('^/dataset[0-9]+/data[0-9]/', quantity_path).group(0)            
                dataset_path = os.path.join(dataset_path, 'data')
                if isinstance(self[dataset_path], h5py.Dataset):
                    self.dataset = self[dataset_path].ref
                    return dataset_path