Improve selectivity estimation for @-Queries

[thomaskrause]

Addresses the issues with the queries in #321

[github-actions]

Commit 7966e8d

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Diff Coverage
Diff: origin/main...HEAD, staged and unstaged changes
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cli/src/bin/annis.rs (0.0%): Missing lines 324,445
core/src/annostorage/inmemory.rs (100%)
core/src/annostorage/ondisk.rs (0.0%): Missing lines 900,920-924
core/src/graph/storage/adjacencylist.rs (71.4%): Missing lines 478-485
core/src/graph/storage/dense_adjacency.rs (100%)
core/src/graph/storage/disk_adjacency.rs (75.0%): Missing lines 177
core/src/graph/storage/disk_path.rs (100%)
core/src/graph/storage/legacy.rs (100%)
core/src/graph/storage/linear.rs (100%)
core/src/graph/storage/mod.rs (97.1%): Missing lines 246
core/src/graph/storage/prepost.rs (100%)
graphannis/src/annis/db/aql/operators/edge_op.rs (68.4%): Missing lines 61-66
graphannis/src/annis/db/exec/mod.rs (100%)
graphannis/src/annis/db/exec/nodesearch.rs (100%)
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Total:   183 lines
Missing: 24 lines
Coverage: 86%
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