Add structured reference and instant payment support

[vincestus]

This PR adds two major features to python-sepaxml:

Structured Description Support

Adds support for structured creditor references in both SEPA Direct Debit and SEPA Credit Transfer transactions:

• ISO 11649 RF references: International structured creditor references (e.g., RF18539007547034)
• BBA structured references: Belgian structured references (e.g., 000/0000/00196)
• Automatic validation with modulo 97 checksum verification
• Proper XML generation with nodes instead of for structured descriptions
• Mutually exclusive handling of structured vs unstructured descriptions

Usage:

payment = {
    "name": "Customer Name",
    "IBAN": "BE68539007547034",
    "amount": 5000,
    # Use either description (unstructured) OR structured_description (not both)
    "structured_description": "RF18539007547034",  # ISO 11649
    # "structured_description": "000/0000/00196",  # BBA format
}

Instant Payment Support

Adds support for SEPA Instant Credit Transfers (SCT Inst) with the instant_payment flag:

• Sets INST in the XML
• Works with all supported PAIN.001 schemas
• Fully tested with multiple batch scenarios

Usage:

payment = {
    "name": "Recipient Name",
    "IBAN": "DE89370400440532013000",
    "amount": 1000,
    "instant_payment": True,  # Enable instant payment
    "description": "Urgent payment",
}

Additional Improvements

• Comprehensive test coverage (677 new test lines)
• Fixed XML ElementTree deprecation warnings
• Fixed pytest return value warnings
• Updated README with examples and documentation

All 45 tests passing.