doc: add some information about the raw_data in CSR format

doc/src/raw_exchange.dox

@@ -221,6 +221,10 @@ accordingly.
       `raw_data` is a data class object, see \ref yac.RawInterpWeights "RawInterpWeights"
 </table>
 
+Alternatively, the exchange data can also be obtained in a format that
+is compatible with the CSR sparse matrix format. See for example \ref
+yac_cget_raw_interp_weights_data_csr.
+
 \subsection src_field_buffer_raw_exchange Allocate receive buffers
 
 <table style="border: none;border-collapse: collapse;width:1500px">
@@ -608,6 +612,17 @@ accordingly.
           for tgt_idx, start_idx, end_idx in zip(raw_data.wgt_tgt_idx, indptr[:-1], indptr[1:]):
             tgt_buffer[c, tgt_idx] = raw_data.weights[start_idx:end_idx]@buf[c, raw_data.src_idx[start_idx: end_idx]]
      \endcode
+
+     In the case you have the raw_exchange data in CSR format (see \ref yac.Field.get_raw_interp_weights_data_csr), you can pass the data directly to scipy's `csr_matrix`.
+
+     \code{.py}
+       raw_data_csr = tgt_field.get_raw_interp_weights_data_csr()
+       W = csr_matrix((raw_data_csr.weights, raw_data_csr.src_idx, raw_data_csr.src_indptr))
+       tgt_field.get_raw(buf)
+       for c in range(tgt_field.collection_size):
+         tgt_buffer[c, :] = W@buf[c, :]
+         tgt_buffer[c, raw_data_csr.tgt_idx_fixed] = raw_data_csr.fixed_values[0]
+     \endcode
 </table>
 
 \subsection cleanup_raw_exchange Freeing of interpolation information