Optimize land methods

* Modified std::sting arguments passed by reference.
* Declared iterators outside the for loops.
Fix miscalculation of LineStringGraphMeanAttribute

* Fixed error due to LineStringEntity getCentroid() method
* Modified related tests
Add method to load AttributeValue from a GeoVector to a LandRGraph

* Added method to load AttributeValue by SelfId.
* Added related tests.
 Add method to load AttributeValue from a GeoVector to a LandRGraph
    
    * Added method to load AttributeValue by Location.
    * Added related tests.
Add method to merge two coincident LineStringEntities

* Added method in LineStringGraph to merge two LineStringEntities
* Added related tests
Add method to obtain list of LineStringEntities which are under a legth
threshold and with specific characteristics

* Added getLineStringEntitiesByMinLength method 
* Added related tests
 Fix bug in method to merge two coincident LineStringEntities
    
    * Fixed bad call of GeometryFactory
    * Updated related tests
Merge branch 'develop' into develop-qt
Preferences dialog in OpenFLUID-Builder with Qt

* Added the preferences management dialog, subdialogs and applying
system
* Added icons for add/edit/remove and move buttons
* Cleaned code
* Fixed small bugs in preferences management

(references OpenFLUID/openfluid#246)