digital: scrambler docs now match the new functionality
digital: OFDM headers now have auto-scramble feature (reduce PAPR spikes)
digital/packet_utils: adding parameter to take a user input preamble to the packet encoder
Digital/pkt: adding ability to dynamically change preamble for pky.py
volk: Add compiler flags for clang in archs.xml

Clang isn't being recognized as gnu, which causes it to build without SIMD instructions.
virtual destructor
Merge branch 'maint'
CID: 1090040. Fairly straight-forward UNINIT fix in pycallback_object.
CID: 1090040. This bug can't, and doesn't need to be, fixed. Documented.
CID: 1088855. Fixing resource leak issues in fcdOpen
CID: 1090068. Fixes missing virtual destructor issue.
Fixing issue CID 1046354.
Fixing CID 1043304. Lol wut?! ROFL
fix: max_noutput_items is set globally but it should be local to a block.
Merge branch 'maint'
digital: packet_header_default now comes with an 8-Bit CRC
digital: Fixed *_ofdm.grc examples (Issue #593)
filter: sets comments for windows to export into Doxygen manual.
filter: fixing windows functions.

Both Kaiser and Blackman windows were wrong. Fixed according to equations in Oppenheim and Schafer. Thanks to Chris Sylvain (KB3CS) for pointing out issues with the Kaiser window.

Added Barlett (triangular) window.
Merge branch 'maint'