Adding multipulse fast r2s example using independent operator

[shimwell]

this should speed up the neutron activation simulations for multiple pulse simulations

using openmc.deplete would make use of the coupled operator which would run a transport simulation for each time step. This makes use of the independent operator so we just need on transport simulation

for the example here we have 7 pulses of 1 second so this method using the independent operator is roughly 7 times quicker than using the coupled operator