GrExtras next -> master

Hey you guysers,

Whats up? With the upcoming gnuradio 3.7 API changes, I am now
deprecating the old master branch of GrExtras. Since that both projects
have a gnuradio/block.h file, making building incompatible – now is
probably a good time to perform the ritual of the cycling of branches.

How about that old code? Anyone using the old master branch of GrExtras
can continue doing so. Git does not forget. The master branch has been
renamed to grextras_v3.6 and can be compiled against GNU Radio 3.6
And the corresponding wiki page for the old master branch is now here:

And that sweet new code? The next branch of GrExtras is now named
master. All features from the old master branch have made it into the
stock scheduler, advanced scheduler, or current GrExtras master branch.
This is the corresponding wiki page and block list:

I hope that this avoids some confusion. Thanks!
-josh