Update XMLImportExport.java

[inonwir]

Refactor XML Export Logic for Stream Reusability and Maintainability

Summary of Changes
This pull request introduces a focused refactor of the XMLImportExport class to improve modularity, code reuse, and maintainability by:

• Extracting XML export logic into a new method: writeToStream(...)
• Updating the existing exportToFile(...) method to reuse the writeToStream(...) logic
  -Preserving all existing behavior with no breaking changes

Motivation
The original export method tightly coupled file output with XML logic. By decoupling this into a reusable writeToStream() method:

• The code becomes easier to test and maintain
• Future developers can extend the export destination (e.g., memory buffer, network stream) without duplicating logic
• Improves separation of concerns between stream writing and file I/O

Technical Details
writeToStream() takes an XMLEventWriter and handles the full RSS/XML generation process, including headers, molecule records, and units

• The exportToFile() method now simply opens a file stream and passes it into writeToStream()
• No logic changes to importFromFile() — only the export path is refactored

Backward Compatibility
This change does not break existing code. The exported files will remain identical, and external consumers using exportToFile() will not be affected.