blocks: Convert GRC examples/msg_passing to YAML format

This commit converts the examples in `gr-blocks` to the new YAML format.
This conversion is tracked in gnuradio#2285.

The flowgraphs in `examples/msg_passing/hier` are converted as well,
though hier block generations seems to be broken. In order to load
`test_msg_hier_topblock.grc` it is necessary to:
1. Open and generate `test_msg_hier.grc` manually
2. Update the generated `test_msg_hier.py.blocks.yml` in the system
`.grc_gnuradio` folder. The input and output IDs are not unique. All
inputs are named `out0` all outputs `in0`. Rename the second input and
output to end in `1` in order to make things work.

Author: jdemel