Remove of deprecated "function" term in OpenFLUID-Market GUI

(references OpenFLUID/openfluid#166)
Switch to version 2.0.0~beta2
Add of generic type for associating units IDs to data/values (IDMap)

(closes OpenFLUID/openfluid#220)
Improvement of robustness of observers

* Added test of ware parameters 
* Raised exceptions with understandable messages
* Cleaned code

(closes OpenFLUID/openfluid#212)
Improvements on builder-extensions execution management

* Added handling of exceptions thrown by builder-extensions
* Added emission of update signal when builder-extension execution
  is completed
* Added builder-extension for testing exception handling  

(closes OpenFLUID/openfluid#204)
(references OpenFLUID/openfluid#218)