add burst tagging, preamble detect, phy, and loopback example

examples/cdc_phy_loopback.grc

examples/cdc_phy_rx.grc

examples/cdc_phy_tx.grc

grc/CMakeLists.txt

grc/cdc_phy_rx.block.yml

grc/cdc_phy_tx.block.yml

grc/cdc_preamble_detect_cc.block.yml

grc/cdc_tag_tx_burst_cc.block.yml

include/gnuradio/cdc/CMakeLists.txt

include/gnuradio/cdc/preamble_detect_cc.h

include/gnuradio/cdc/tag_tx_burst_cc.h

lib/CMakeLists.txt

lib/preamble_detect_cc_impl.cc

lib/preamble_detect_cc_impl.h

lib/tag_tx_burst_cc_impl.cc

lib/tag_tx_burst_cc_impl.h

python/cdc/CMakeLists.txt

python/cdc/__init__.py

python/cdc/bindings/CMakeLists.txt

...c/bindings/docstrings/preamble_detect_cc_pydoc_template.h