NxtomoReader: read NXTomo files without DataUrls

About

This MR is a tentative simplification of the data reading process for HDF5-NXTomo files.

Currently, ChunkReader uses a list of DataUrl to browse the files, which is cumbersome and inefficient.

NXTomoReader will load the data directly from entry/measurement/data (or simiar) and bypass the DataUrl collection.

To do

• Base NXTomoReader
• Advanced features
  • Arbitrary Sub-region
  • Read with read_direct()
  • Support on-the-fly processing function
• Unit tests

Notes

When a processing function is passed, the memory usage can be up to twice bigger, since the reader does something like

    dptr = file_desc[data_path]
    dptr.read_direct(dst_buffer, source_sel=...)
    self.data = self.processing_function(dst_buffer)