Use enumeration type in Markierungsalgorithmus

[jvoigtlaender]

Instead of 2, 1, ... a proper custom type should be used here:

logic-tasks/flex/markierungsalg.flex

Lines 226 to 228 in ceefd0d

	2 -> ("\\"unerfüllbar\\"","\\"unsatisfiable\\"")
	1 -> ("\\"erfüllbar\\"","\\"satisfiable\\"")
	_ -> ("","")