Fix bug in IndexTable::printStatistics

The code that computes the top_N elements is bugged - it simply find the
topmost elements that is not greater than the current one, and then it
overwrites it. The correct behaviour should be instead that once we
decide we are better than element j, that element shoulc be moved to j+1,
and j+1 should be moved to j+2 and so on.

This can be easily fixed with std::move_backward, to ensure the elements
are moved forward from the last to the first to be moved.

If in the future we'd like to make this more efficient, we could implement
this using a heap instead.

This is (part of) the log we get when calling createindex on the example
data, prior to this fix:

Index statistics
Entries:          8552346
DB size:          537 MB
Avg k-mer size:   0.133630
Top 10 k-mers
    GQQVAR      190
    QLGQRV      110
    IHDKNI      105
    ALGSGK      105
    LLPGKT      102
    SGGTLR      84
    SGLGRV      75
    VGSSST      61
    VMHAGS      59
    ATADTT      59

And this is after this fix:

Index statistics
Entries:          8552346
DB size:          537 MB
Avg k-mer size:   0.133630
Top 10 k-mers
    GQQVAR      190
    GPGGTL      134
    SGQQAI      112
    QLGQRV      110
    GKTLIA      106
    GGKRIA      106
    IHDKNI      105
    ALGSGK      105
    LLPGKT      102
    QRRARA      94

Author: lukecavabarrett

src/prefiltering/IndexTable.h

@@ -276,6 +276,7 @@ class IndexTable {
                 continue;
             for (size_t j = 0; j < top_N; j++) {
                 if (topElements[j].first < ((size_t) size)) {
+                    std::move_backward(topElements+j, topElements+top_N-1, topElements+top_N);
                     topElements[j].first = static_cast<unsigned long>(size);
                     topElements[j].second = i;
                     break;