openmc plot mode used for speed

[shimwell]

This is another option for speeding up the slicing.

This runs openmc in plot mode and loads up the resulting ppm into a 2d pixel array.

The main disadvantage is that we currently need nuclear data when running in plot mode. Perhaps this is something that can be changed in openmc

TODO

• cell id slicing function
• include nuclear data in repo as datafile
• include cross_sections.xml in repo as datafile
• make a minimal h5 data file from an empty ACE file (or use He4 which is a small h5 file)

handy function for timing the method to see the speed improvement

import time
for x in range(2):
    start_time = time.time()
    data_slice = my_geometry.get_slice_of_material_ids(view_direction='x')
    print("--- %s seconds ---" % (time.time() - start_time))

method is about 10 times quicker than current method for tokamak example