make normalize return type match base type

alib2data/src/alphabet/SymbolBase.h

alib2data/src/automaton/AutomatonBase.h

alib2data/src/grammar/GrammarBase.h

alib2data/src/label/LabelBase.h

alib2data/src/regexp/RegExpBase.h

alib2data/src/rte/RTEBase.h

alib2data/src/string/StringBase.h

alib2data/src/tree/RankedTreeBase.h

alib2data/src/tree/TreeBase.h

alib2data/src/tree/UnrankedTreeBase.h

alib2data/src/tree/ranked/PrefixRankedBarNonlinearPattern.h

alib2data/src/tree/ranked/PrefixRankedBarPattern.h

alib2data/src/tree/ranked/PrefixRankedBarTree.h

alib2data/src/tree/ranked/PrefixRankedNonlinearPattern.h

alib2data/src/tree/ranked/PrefixRankedPattern.h

alib2data/src/tree/ranked/PrefixRankedTree.h

alib2data/src/tree/ranked/RankedNonlinearPattern.h

alib2data/src/tree/ranked/RankedPattern.h

alib2data/src/tree/ranked/RankedTree.h

alib2data/src/tree/unranked/PrefixBarTree.h