aconversion: Use list::front() and back() instead of iterators

aconversions/src/fa2rg/fa2lrg/FAtoLRGConverter.cpp

aconversions/src/fa2rg/fa2rrg/FAtoRRGConverter.cpp

aconversions/src/rg2fa/lrg2fa/LRGtoFAConverter.cpp

aconversions/src/rg2fa/rrg2fa/RRGtoFAConverter.cpp

aconversions/src/rg2re/lrg2re/LRGAlgebraic.cpp

aconversions/src/rg2re/rrg2re/RRGAlgebraic.cpp

aconversions/src/rg2rg/lrg2rrg/LeftToRightRegularGrammar.cpp

aconversions/src/rg2rg/rrg2lrg/RightToLeftRegularGrammar.cpp