CSV file observer

* Completed observer for exporting variables to CSV files
* Added simple test function for production of variable at variable time
steps

(references #18)
Enhanced CSV observer for handling asynchronous variables

* Added simulation function for tests, producing variables values at
fixed time
* Added methods in ValuesBuffer and Variables class, allowing to get
values with a time index condition

(references #18)
Cleaning of unused source code and update of tests

* Removed deprecated simulation functionalities for fixed time step
* Removed deprecated file output system
* Removed deprecated tests and tests datasets for file output system
* Renamed tests

(references #18, #5)