Support for camt.054 / document type autodetection / minor updates/fixes

[MarcoSteinacher]

We've been using the library for a while now for camt.053 files. Now we needed support for camt.054, which I have added here. Along the way we also added auto-detection of the document type and updated/fixed a few other things to make it work for our documents (Swiss banks).

Major changes:

• Added support for camt.054 (bank_to_customer_debit_credit_notification)
• Auto-detection of document type from xml namespace: If parse() is called with structure=None, the matching structure is selected automatically based on the xmlns attribute of the document and the standard and compatible_standards attributes of the sepa_messages objects. If no matching structure is found, the UnsupportedDocumentType exception is raised. The document type is returned as document_type in the data.
• Better support for 2019 versions of camt (v08): Added party_compat(), which should make the parser compatible with v06 and v08 documents. Added end_to_end_uuid.

Minor changes:

• Added 'Ownr' to account()
• Added 'AddtlInf' to statement_group_header()
• Implemented structure for 'Chrgs' in charges()
• Implemented structure for 'Strd' in entry() [remittance_information]
• Use etree.QName(tree).localname to get the tag name without the xmlns prefix.
• Fixed typos: 'ClrSysMmdId' > 'ClrSysMmbId', 'AddtInf' > 'AddtlInf'
• Added schemas for v04 and v08 of camt.053 and camt.054

I would be happy to contribute those changes to the main repository.

Note: It is not clear to me what the best way to support multiple versions of a document type (e.g. camt.053.06 and camt.053.08) at the same time. Currently it seems to works, but it doesn't seem to be very clean or consistent. Is there a strategy to address that in the future?