Digital-rf

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This is primarily a bugfix release. Changes include:

- Add raster and vector tools to gr_digital_rf for working with periodic data.
- Disable file locking with HDF5 >= 1.10 for digital_metadata, which restores behavior so it matches that of HDF5 1.8.
- Fix error using digital_rf_sink with GNU Radio 3.7.12, which changed the type of its input and output signature objects.
- Fix the digital_rf_sink block in GRC to correctly pass an empty center frequency.

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The main impetus for this release is a complete reorganization of the build system to enable Digital RF to run with Python 3 and on Windows. Major changes include:

- Python 3.5+ compatibility (excluding 'gr_digital_rf' since GNU Radio does not support Python 3 yet).
- Windows compatibility, including conda packages.
- Automated CI testing with revamped Python tests using 'pytest'.
- Python package available on PyPI (for 'pip' install), including binary wheels for Linux, OSX, and Windows.
- Python 'gr_digital_rf' packaged with 'digital_rf' since we no longer require GNU Radio to build (dropping Digital RF C Sink support).
- New 'thor' options:
- Output channel settings including rational resampling, frequency shifting, and channelizing
- Clock source (10 MHz ref) and time source (PPS) split out from sync_source
- Clock lock check with nolock option to skip
- LO source/export
- Tuning arguments
- DC offset and IQ balance
- Updated sounder example transmit script with some 'thor' features.
- Matlab reader fixes for recent data format and packaging as a Matlab Toolbox.
- Various bug fixes and improvements (see commit log for full list of changes).

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This release incorporates many robustness improvements and fixes based on testing with the Millstone Hill radar. Major changes include:

- Ringbuffer/mirror/watchdog code are now more efficient and robust to errors.
- New 'drf cp' and 'drf mv' commands for copying and moving data.
- The watchdog and cp/mv commands now support specifying a start and end time to watch/copy/move only a particular window of time.
- Many fixes and updates to the beacon example.
- Added ability to specify input/output chunksize in the GNU Radio Digital RF Sources/Sinks in order to tweak performance for a particular application.

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Improvements to watchdog_drf, list_drf, mirror, and ringbuffer. Can now monitor directories that don't yet exist or get deleted and ringbuffer by file count and duration.

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Fix build on OSX for C version of gr_drf Digital RF Sink.

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The main new feature is a GNU Radio Digital RF Sink written entirely in Python that writes receiver and recorder metadata previously handled only in the thor recording script.