introduce AnaAlgorithm::efficiency() as alias for AnaAlgorithm::histeff()

Merged Nils Erik Krumnack requested to merge krumnack/athena:AnaAlgorithm_efficiency into master

It was pointed out that the name in AnalysisBase wasn't matching the name in Athena, so I added an alias in AnalysisBase. I kept the old version around because removing it would break users code.

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