AnalysisTop: Add option to filter nominal branches

Tomas Dadorequested to merge
tdado/athena:21.2-AT-NominalFilter into 21.2

This MR adds a new config option that allows to filter the nominal tree branches. This can be used together withthe general filtering option to apply a different filtering on nominal and on the systematic branches.

Addresses the suggestion from ANALYSISTO-1079

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