AST-1249 Simplify solution index in DDECal
Remove SolutionInterval::NSolution and use DDECal::itsFirstSolutionIndex and solution_index instead. Also remove unused DDECal::itsNSolInt.
Remove SolutionInterval::NSolution and use DDECal::itsFirstSolutionIndex and solution_index instead. Also remove unused DDECal::itsNSolInt.