Grainstats entry point

[ns-rse]

Further to #517 add an entry point for topostats grainstats which loads files saved as .topostats (handled by io.LoadScans class) and calculates statistics.

Questions...

• Should the statistics be saved as part of the HDF5 data? I ask because in batch processing end-to-end the grain statistics are often combined with the tracing statistics so if a user were to load a .topostats file to perform tracing they might expect to have grain statistics combined. Of course if they haven't undertaken grain statistic calculation and jump straight to tracing and tracing statistics this will never be possible.