gr_modtool: Swiss Army Knife for editing GNU Radio modules and -components.

Adding a new subcommand for Command Line Interface
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* Add a new file called SUBCOMMAND in the cli directory
* Create a function cli with the decorator @click.command or @click.group
* Add the necessary options for the command or command group
* Add that file to __init__.py and CMakeLists.txt in the cli directory


Adding a new subcommand for Exposing as an API
==============================================

* Add a new file called SUBCOMMAND in the core directory
* Have a look at the other subcommands, it must inherit from ModTool
* Add that file to __init__.py and CMakeLists.txt in the core directory


The gr-newmod directory inside the templates directory
======================================================

This dir basically contains a copy of gr-howto-write-a-block from the gnuradio
sources, with some differences:
- All example blocks, apps, grc files (...) and references thereto in the
  CMake files are removed
- In the top-level CMake file, the project is called 'gr-howto'.
- Any time anything relevant is changed in gr-howto-write-a-block, it should
  be changed here, too.