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

Adding a new subcommand
=======================

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

The gr-newmod 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.