issue #43: sharded DBUtils
issue #43: fixed a bug related to aggregation on strings
issue #43: added tests for more operations
issue #43: fixed a broken commit
issue #43: removing variant tables on shards when the project is removed
issue #43: sharded removal of references
issue #43: addedcode for file manipulation
issue #43: sharded import of TSV files, fixed several bugs related to export/import
issue #43: extended tests for write operations