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

src/File.cpp

@@ -73,10 +73,11 @@ size_t File::tell() const { return file_impl->tell(); }
 size_t File::rows() const { return file_impl->rows(); }
 size_t File::cols() const { return file_impl->cols(); }
 size_t File::bitdepth() const { return file_impl->bitdepth(); }
+Dtype File::dtype() const { return file_impl->dtype(); }
 size_t File::bytes_per_pixel() const {
     return file_impl->bitdepth() / bits_per_byte;
 }
 
 DetectorType File::detector_type() const { return file_impl->detector_type(); }
 
-} // namespace aare
+} // namespace aare