ListIterators tested.
SortMerger added.
Modified a bit.
Added bit of code, to be sure we run with assert() handled.
Improved (I think so) merger.
MergeSorter modified.
TupleComparator extracted from ExternalSort to Utils.
New SortMerger class added, using internal buffer.
SortMerger concrete test class uses limited buffer size.

#TODO: SortMerger is not yet aware of that.
SortMerger assumes infinite size of accessible pages in buffer.

This could be overcome, if the second input equival tuples is no larger than
the buffer size (Page size). Otherwise, one of the buffers has to be increased.
OperatorTuplesIterator extracted from ExtenralSort.
Modified RelationIOManager so that iterator may be created even if numPages==0.
Small modifications to SortMerger2.
MergeJoin implemented.