Provide flattened XSD file for tools that cant work with include

[roman-dvorak]

It has been discussed that some users prefers to have a single large XSD file, because some tools have problems with working with included XSD sub-models.

to solve this issue i earlier modified xsdmergerh

With this tool, it is possible to automatically merge all XSD files into one flat XSD file (e.g. using GitHub Actions in this repository), which these users could then use.

The resulting XSD file passes the xmllint validation (after solving #92):

> xmllint --noout --schema dataset/schema.xsd ccmm_sample.xml
ccmm_sample.xml validates

> xsdflatten --output flattened_ccmm.xsd dataset/schema.xsd

> xmllint --noout --schema flattened_ccmm.xsd ccmm_sample.xml
ccmm_sample.xml validates

The output file is attached: flattened_ccmm.xsd

[mesemus]

Please add a github workflow triggered on every commit that would:

1. check the xsd files if they are correct
2. flatten them to a single xsd file
3. make sure that the example file/files can be validated.
4. commit the flattened file if it has changed

This way we would know that the CCMM repository is in consistent state and implementers can rely on that.