blocks: Added tagged stream mux feature to preserve "head" position of tags on other inputs than 0
digital: carrier allocator tag propagation now handles sync words properly
digital: OFDM Tx hier block preserves head tag position
digital: Update OFDM examples to use new features
fft: Moved all window building functions into C++.

Remove window.py. All windows built using C++ functions. Exported into Python in same module (from gnuradio.fft import window).

Removed all window building work in firdes, too. firdes.window now makes a direct call to fft.window.build with same parameters.

Added documentation for window functions, including references to where to find coefficients and equations for (most of) the windows.

All changes should not affect existing code.

f
Merge remote-tracking branch 'martin/tag-propagation'
bug fix in pdu_to_tagged_stream_impl.cc

d_remain of floats would never empty
runtime: add qa for fast_atan2f, removed a float equality test
Merge branch 'maint'
Merge remote-tracking branch 'tom/windows'
Merge branch 'master' into next