Prototype multi threaded file reader (#343)

- Reading files from nfs shares tops out at ~1GB/s for single threaded
reads.
- MultiThreadedFileReader uses our File wrapper to read a generic file
in parallel
- Placed in aare::experimental to show that it's not production ready

docs/src/MultiThreadedFileReader.rst

@@ -0,0 +1,37 @@
+MultiThreadedFileReader
+=======================
+
+``MultiThreadedFileReader`` reads one chunk per worker on each call. Each
+worker owns an independent :cpp:class:`aare::File`, while all workers write
+into non-overlapping regions of the destination buffer. The output is ordered
+by frame index and successive calls advance through the file.
+
+.. code-block:: cpp
+
+   #include "aare/MultiThreadedFileReader.hpp"
+
+   // Four workers, chunks of 128 frames, and at most 10,000 frames.
+   aare::experimental::MultiThreadedFileReader reader(path, 4, 128, 10'000);
+   while (reader.remaining_frames() != 0) {
+       // Contains at most 4 * 128 frames.
+       auto batch = reader.read();
+       process(batch);
+   }
+
+Omit the final argument to read every frame in the source. An explicit value
+of zero requests an empty result. The low-level ``read_into`` overload avoids
+an allocation when the caller already owns a buffer of at least
+``reader.next_read_bytes()`` bytes. Use ``read_all()`` to read every frame
+remaining from the current position, and ``seek()`` to reposition the reader.
+Call ``close()`` to release all worker file handles early.
+
+.. note::
+
+   Multiple workers do not guarantee faster reads. Performance depends on the
+   storage device and file format, so the thread count and chunk size should be
+   benchmark-driven.
+
+.. doxygenclass:: aare::experimental::MultiThreadedFileReader
+   :members:
+   :undoc-members:
+   :private-members: