Various improvements to the sample GRC flowgraph

Hi Bastian,

As we discussed by email, I made various improvements to the sample GRC flowgraph in gr-rds.  I wasn't able to track down the source of the RDS errors I'm seeing on some stations, so this pull request doesn't include any fixes for that.  I'll send another pull request if I come up with something.

There may still be more optimizations that can be made to CPU utilization by increasing the transition width of excessively sharp filters, but I haven't looked at that yet.

Another thing that might be worth looking into is ensuring that the L+R and L-R signals stay synchronized.  Right now I suspect there is a delay between them because there are more filters in the L-R signal path.  Perhaps filter delay blocks could be used to fix this.

- Clayton