Improvements on tests and CMake configuration cleaning

* Added test script and macro for the execution of actions 
  before and after the test command
* Refactored CMake modules
* Updated tests

(references #36)
Cleaning of Vtk observer

* Removed debugging info on stdout

(references #53)
LandRGraph and children now deal with Datasets

* LandRGraph and children now can be created from a VectorDataset
* Adding a raster to a LandRGraph can now be done with a RasterDataset
* VectorDataset internal filename is now suffixed with timestamp, allowing creation of many VectorDatasets from the same GeoVectorValue source
* Added Raster/VectorDataset copy constructor

(references #41)
Merge branch 'develop' of https://github.com/OpenFLUID/openfluid into refactor-geovalues