This function is called by fprintf_network to write the maps part
of a network file, i.e. the part between MAPBEGIN and MAPEND.
See NET-IO-format for the layout of this part. Unlike
fprintf_layers, this routine writes the maps for one layer only, so
it has to be called for every layer in the network.
NOTE
Since layer- and map-information is only stored for shared weights
neural networks (having the NET.NetFlag field set to SHAREDNET)
this function is only called when the NetFlag field has that value.
