change std::variant to have specific constructors

alib2data/src/automaton/AutomatonFromXMLParser.cpp

alib2data/src/automaton/FSM/EpsilonNFA.cpp

...ata/src/automaton/FSM/FiniteAutomatonFromStringParser.cpp

alib2data/src/automaton/PDA/NPDA.cpp

alib2data/src/automaton/PDA/SinglePopNPDA.cpp

alib2data/src/std/variant.hpp

alib2data/test-src/std/StdTest.cpp

alib2data/test-src/std/StdTest.h