[release] 0.6.0 - Merge develop to master

[release] 0.6.0

[release] 0.5.2

[release] release 0.5.2

[#1419] prepare release 0.5.1 (#1421)

fixes #1419

[release] 0.5.0

[release] 0.5.0

Merge pull request #1228 from dbs-leipzig/develop

[release] v0.4.5

[release] release 0.4.4

[release] release 0.4.4

[release] release 0.4.4

[release] release 0.4.4

[release] version 0.4.3

[release] version 0.4.3

[release] version 0.4.2

[release] version 0.4.2

[release] release 0.4.1 (#957)

### Release 0.4.1

* [#877] added support for property value null (#908)

fixes #877

* [#856] property pushdown hbase (#906)

fixes #856

* [#864] Change examples to use csv (#905)

fixes #864

* [#494] Add Set support to PropertyValue (#916)

fixes #494

* [#888] Minor changes to Tuple-related functions. (#912)

fixes #888

* [#779] Add code coverage for Gradoop (#914)

fixes #779

* [#702] Add support for graphs without edges to TLFDataSource (#921)

fixes #702

* [#909] Remove Flink dependency for store instance (#918)

fixes #909

* [#876] added print for logical graph and graph collection (#913)

fixes #876

* [#857] logical predicate chaining hbase (#920)

fixes #857

* [#926] added argLine to maven properties (fix ide testing bug) (#929)

fixes #926

* [#794] include new benchmark classes (#904)

fixes #794

* [#312] example program for aggregation (#925)

fixes #312

* [#901] Add SamplingConstants, AggregationConstants (#934)

fixed #901

* [#935] Add tests for containment functions (#941)

fixes #935

* [#900] Refactor sampling tests. (#907)

fixes #900

* [#936] Generalized RollUp and DrillDown (#939)

fixes #936

* [#947] Fix PageRank wrapper and add a test. (#949)

fixes #947

* [#942] graph density computation for sampling evaluation (#946)

fixes #942

* [#954] prepare minor release (#955)

fixes #954