Fix issue with he detectors

[mcolomermolla]

This should close issue #205

The problem was the following:

• guess_material() was not considering the '_he' in the material name, which was causing that efficiency and smearing files were not found, and those matrices were set to ones. This was producing no error in the ar40kt_he case, but it was not properly running with he eff and smear files.
• for the case of wc100kt30prct_he detector, the oxygen files don't contain '_he' in the channel, but ibd and nu-e ES do. So for the oxygen case, the file name was correctly found, but there was a missmatch with the rates binning, which was set only for the non he case.

[github-actions[bot]]

Result of Benchmark Tests

Benchmark	Min	Max	Mean
python/snewpy/test/test_snowglobes.py::test_simulation_chain_benchmark	5.18	5.30	5.24 +- 0.05