Use the model descriptor to write fluidx
Use the domain descriptor to write fluidx
Write <model> tag in fluidx even if model is empty

* Write empty <model> section in fluidx file if model descriptor is empty
* Cleaned code
Use the datastore descriptor to write fluidx

* Used the datastore descriptor to write fluidx file(s)
* Written an empty <run> section if run descriptor isn't filled
* Cleaned code
Monitoring section is now written to file(s)

* Added writing of <monitoring> section in fluidx file(s)
* Added check of <monitoring> section in FluidXDescriptor tests (Monitoring is now part of all test input dataset, specific "with observers" test has been removed)
* Added global check of written files
* Fixed missing writing of run <simid> section
* Renamed fluidx test dataset from FluidXReader to FluidXDescriptor
Reactivate save function in Builder