Allow to execute AQL directly on loaded AnnotationGraph objects.

[thomaskrause]

This API is an alternative for using a CorpusStorage when only one corpus is handled and adds the new functions aql::execute_query_on_graph and aql::parse functions.

use graphannis::*;

let graph = AnnotationGraph::with_default_graphstorages(false)?;
let query = aql::parse("tok", false)?;
let it = aql::execute_query_on_graph(&graph, &query, true, None)?;
assert_eq!(0, it.count());

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (fb32b9d) 65.04% compared to head (cd63bf7) 65.10%.

Files	Patch %	Lines
graphannis/src/annis/db/aql/mod.rs	93.10%	1 Missing and 1 partial ⚠️
graphannis/src/annis/db/aql/disjunction.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #278      +/-   ##
==========================================
+ Coverage   65.04%   65.10%   +0.05%     
==========================================
  Files          75       75              
  Lines       19015    19044      +29     
  Branches    19015    19044      +29     
==========================================
+ Hits        12369    12399      +30     
  Misses       5304     5304              
+ Partials     1342     1341       -1     

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