avoid calling cif::Category::size() too often

include/cif++/Cif++.hpp

@@ -2152,6 +2152,7 @@ class Category
 	std::vector<ItemColumn> mColumns;
 	ItemRow *mHead;
 	ItemRow *mTail;
+	size_t mLastUniqueNr = 0;
 	class CatIndex *mIndex;
 
 	std::vector<Linked> mParentLinks, mChildLinks;

src/Cif++.cpp

@@ -1601,11 +1601,13 @@ std::string Category::getUniqueID(std::function<std::string(int)> generator)
 	if (mCatValidator != nullptr and mCatValidator->mKeys.size() == 1)
 		key = mCatValidator->mKeys.front();
 
-	size_t nr = size();
+	// calling size() often is a waste of resources
+	if (mLastUniqueNr == 0)
+		mLastUniqueNr = size();
 
 	for (;;)
 	{
-		std::string result = generator(int(nr++));
+		std::string result = generator(static_cast<int>(mLastUniqueNr++));
 
 		if (exists(Key(key) == result))
 			continue;