Added postprocess() and run() methods to Benchmark class

[SteSeg]

MERGE AFTER PR #57
In OpenmcBenchmark class the postprocess() method opens an openmc statepoint file, a mesh.h5m file (via pydagmc) and the benchmark specifications file. It retrieves all the tallies in the statepoint file, normalizes them over surface area or cell volume, if necessary (with the actual area/volume measure provided in the mesh file), converts them into xarray.DataSet and store them in a benchmark_result.h5 file.

The run() method wraps the openmc.run() method and calls the postprocessing() method right after. To run a benchmark simulation should be as easy as:

import openmc_fusion_benchmarks as ofb

ok = ofb.OpenmcBenchmark(name='oktavian_al')
ok.run()