Refactor ExperimentExporter

ExperimentExporter is refactored into separate functions for clarity and avoiding accidental bugs; furthermore,

• BLOSC filter initialization and release is moved to a class with proper c'tor and d'tor (RAII principle).
• HDF5-related DataType and DataSpace defintions are unified when possible.
• Header inclusion (H5Cpp.h) is moved to .cpp file, as it is not needed for the interface declaration.
• Code repetitions are removed.
• The structure of the .nsx file is clarified (core/experiment/nsx_format_definition.txt).