This is a follow up of PR #1244: the destructor FGDistributor::Case::~Case is not deleting its member Test because the call to delete Test is missing. It should have been:

    ~Case() {
      for (auto pair: PropValPairs) delete pair;
+     delete Test;
    }

Given this and the leak that has been fixed in PR #1244, this PR is addressing the issue upfront and replaces all pointers in FGDistributor by smart pointers (namely std::unique_ptr<>) which should fix all sources of memory leakage in FGDistributor.

The opportunity is taken to rename the iterator methods of the class FGDistributor::Case from IterPropValPairs, EndPropValPairs to the standardized names begin, end. This allows using a for-range loop instead of an explicit for loop:

-        for (auto propVal = Case->IterPropValPairs(); propVal != Case->EndPropValPairs(); ++propVal) {
+        for (const auto& propVal: *Case) {

which is easier to read.

Finally, the methods FGCondition::Evaluate and FGCondition::PrintCondition are constified to ensure that they are not having side effects on the instances of the class FGDistributor::Case.

Please note that the class FGDistributor::Case is private to FGDistributor and modifying its API has no side effects beyond the internal code of FGDistributor.