Spead2

- Support for pcap files. Files passed to :program:`spead2_recv.py` are now
assumed to be pcap files, rather than raw concatenated packets.
- Only log warnings about the ringbuffer being full if at least one stream
reader is lossy (indicated by a new virtual member function in
:cpp:class:`spead2::recv::Reader`).

- Change :program:`spead2_send.py` and :program:`spead2_send` to interpret
the :option:`!--rate` option as Gb/s and not Gib/s.
- Change send rate limiting to bound the rate at which we catch up if we fall
behind. This is controlled by a new attribute of
:class:`~spead2.send.StreamConfig`.
- Add report at end of :program:`spead2_send.py` and :program:`spead2_send`
on the actual number of bytes sent and achieved rate.
- Fix a race condition where the stream statistics might only be updated after
the stream ended (which lead to unit test failures in some cases).

- Report statistics when :program:`spead2_recv.py` is stopped by SIGINT.
- Add --ttl option to :program:`spead2_send.py` and :program:`spead2_send`.

- Explicitly set UDP checksum to 0 in IBV sender, instead of leaving
arbitrary values.
- Improved documentation of asyncio support.

- Support for asyncio in Python 3. For each trollius module there is now an
equivalent asyncio module. The installed utilities use asyncio on Python
3.4+.
- Add :attr:`spead2.recv.Stream.stop_on_stop_item` to allow a stream to keep
receiving after a stop item is received.
- Switch shutdown code to use atexit instead of a capsule destructor, to
support PyPy.
- Test PyPy support with Travis.

- Remove :option:`!--bind` option to :program:`spead2_recv.py` and :program:`spead2_recv`.
Instead, use :samp:`{host}:{port}` as the source. This allows subscribing to
multiple multicast groups.
- Improved access to information about incomplete heaps
(:py:class:`spead2.recv.IncompleteHeap` type).
- Add :py:attr:`.MemoryPool.warn_on_empty` control.
- Add warning when a stream ringbuffer is full.
- Add statistics to streams.
- Fix spead2_send.py to send a stop heap when using :option:`!--heaps`. It was
acccidentally broken in 1.2.0.
- Add support for packet timestamping in mcdump.
- Return the previous logging function from :cpp:func:`spead2::set_log_function`.
- Make Python logging from C++ code asynchronous, to avoid blocking the thread pool
on the GIL.
- Upgrade to pybind11 2.2.1 internally.
- Some fixes for PyPy support.