blocks: better handling of srcid with vector_source blocks.

 - vector_source now adds the srcid of the tag to the actual tag stream.
 - tag_utils' python_to_tag can now be called with just 3 parameters by omitting the srcid; gets filled in with PMT_F.
 - updated vector_source_with_tags example to have tags with and without srcid set.
grc: get regular variable values directly from param