Modification of the OPENFLUID_SetVariable primitive

Avoiding modifications of past variables
* Removed time parameter in the OPENFLUID_SetVariable primitive
* Updated pass-by-reference of parameters in
  SimulationContributorWare class
* Updated tests and examples

(references #73)
Enhanced primitives for getting values or values lists

* Updated encapsulation of IndexedValue class
* Updated Variables and ValuesBuffer classes
* Added OPENFLUID_GetVariable, OPENFLUID_GetLatestVariable, 
  OPENFLUID_LatestVariables and OPENFLUID_GetVariables primitives
* Updated tests  
* Cleaned code

(closes #73)