Adapt Builder to function params storage as ptree

* Adapted Builder to function parameters storage as boost::ptree
* Fixed merge parameters function, now can deal with multi-level parameters
Add of a CMake variable to enable/disable documentation build
Previous time index access for simulation functions and observers

* Added OPENFLUID_GetPreviousRunTimeIndex() primitive
* Adapted simulation engine to provide previous run information
  to wares
* Updated tests for time primitives
* Cleaned time and durations types definitions

(closes #71)
Buddy to install OpenFLUID examples

* Added example buddy, including basic functionalities 
  for installing examples
* Updated buddies tests
* Added RuntimeEnvironment entries for examples
* Cleaned buddies source code

(references #20, references #80)
Rename and move Builder descriptor classes

* Moved Builderxxx from guicommon to fluidx namespace
* Renamed Builderxxx to AdvancedxxxDescriptor
* Moved GeneratorSignature and FunctionSignatureRegistry from guicommon to ware namespace
Change management of Observers signatures and check

* Created an ObserverSignatureRegistry class to store Observers plugin signatures
* Created a WaresHelper class to perform checks on Monitoring plugin files
Move Function plugins checking to WaresHelper
Refactor FunctionSignatureRegistry

* Made FunctionSignatureRegistry update himself in constructor, avoiding some explicit updates
* Removed FunctionSignatureRegistrySub class, thaht was used for tests but is no more useful
* Used of built-in example functions as default functions for FunctionSignatureRegistry test cases
Change return parameter of WaresHelper check functions

* Monitoring and Model plugins checking now fill a list of missing IDs, instead of a textual list
Refactor relations management in AdvancedDomainDesc

* Added check of Unit relations when adding a Unit to the domain
* Made const the access to Unit relations to forbid direct modifications
* Added functions to add or remove a relation between two Units, and clear all relations of a Unit with others