grc: add python snippets to GRC

[mormj]

This feature adds the ability to insert arbitrary code into the python
flowgraph. It gives a little more low-level flexibility for quickly
modifying flowgraphs and adding custom bits of code rather than having
to go and edit the generated py file

One example is synchronizing multiple USRP objects - sometimes you want
different sync than what is offered in the multi-usrp object, so you can
put a bit of code in the snippet block to do the custom synchronization

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This supercedes #2779 and #2814 - hopefully the issues identified on those PRs have been sufficiently resolved, as the details of last iteration were worked out with @dkozel and @marcusmueller at the ESA hackfest.

• Went back to a single snippet block. This way they can be easily arranged and copied/pasted
• Priority was added to the block so multiple snippets added to the same section can be sorted by priority
• Reduce the possible placements in the rendered flowgraph to inside the main function (can add more later if requested)
• Cleaned up the rendered flowgraph to put the snippet code within their own functions

[marcusmueller]

This is an adequate, good implementation!

It balances two interests:

1. ease of use – the snippet block is as easy as users would want it to have, and it's non-singular
2. sanity of use – the snippets are aggregated in dicts, have order and are inserted into the generated Python on demand.

I think we should let this balloon launch and see how it flies.

[devnulling]

Looks great! I'm going to test out this branch shortly, will merge if no issues found.

[devnulling]

@mormj Can you please add py_snippets_demo.grc to the install list for cmake here: https://github.com/gnuradio/gnuradio/blob/master/gr-blocks/examples/CMakeLists.txt

[michaelld]

I haven't tested / verified functionality, but this PR reads well & I love the concept of inserting snippets ... how many times have I just edited the resulting Python to do the snippets by hand? I'm with Marcus: let's get this merged & see how it flies!

[devnulling]

With the cmake fix mentioned in the comment above, this should be good to go.

[mormj]

@devnulling thanks for staying on top of the examples - too easy to forget. Should be good to go now.

[michaelld]

@devnulling please go ahead and merge if this meets your change request

[devnulling]

Thanks!