Improve export of local feature references

[clausnagel]

This PR proposes some improvements to the export of feature references:

• Global feature references (val_reference_type == 2) are never resolved but only exported as Reference of type GLOBAL_REFERENCE. This is a result of #9 but is differently implemented in this PR.
• Local feature references (val_reference_type == 1) can optionally be resolved. A corresponing option has been added to ExportOptions. This new option is true by default so that local references are resolved.
• When exporting local feature references, the code has been improved to better detect cycles. If there are multiple references to the same local feature, the feature is only exported once. All other references are exported as Reference of type LOCAL_REFERENCE.