Set up schema validation on github worfklows

[Mikejmnez]

Draft PR

This PR:

• Update dap4 schema in this repo to match that in http://xml.opendap.org/dap/dap4.xsd taking this dap3.3.xsd as starting point. Other ones are outdated and incomplete.
• Refactor
• sets of github workflow validation tests.

Action Items for this PR:

• Inspect the D4ParserSax2 to see how things are being defined. This is the reference standard. We can identify which elements are not used and what elements are missing.
• Remove SharedDimension Types
• Remove Array Types
• Remove Alias
• Look at Attributes and how the values are defined. See DAP4: Attribute values that conform to the DAP4 specification are not read correctly. libdap4#259

What is covered in the schema:

• Dataset declaration (what is required). Dimensions
• Attributes (all value types i.e. atomic, String, URI), and the different ways attributes may be defined i.e. containers, multiplevalued, see DAP4: Attribute values that conform to the DAP4 specification are not read correctly. libdap4#259) . See test files in tests/data/Attributes*.dmr
• BaseTypes (Dim declarations, atomic types, etc)
• Enumerations
• Sequences
• Structures
• Groups (dimensions an other attributes)
• Order in which all elements above may be defined (Groups at the very last in every node).
• Opaque types are not included for now. I need an example for that.

@jgallagher59701 @ndp-opendap

[Mikejmnez]

EDIT:
How to validate in Python this simple test dmr using the published schema as shown below:

from lxml import etree
# import requests
# from io import StringIO

dap4_schema_gh = "<path_to_local_dev_branch>/XML/dap4/dap4.xsd"
xmlschema_doc = etree.parse(dap4_schema_gh)
xmlschema = etree.XMLSchema(xmlschema_doc)
#

dmr_test = "<path_to_pydap_test_dmrs>/SimpleGroup.dmr"

f_tests = open(dmr_test)
doc = etree.parse(f_tests)
### now Attempt to validate:
xmlschema.assertValid(doc)

current stage: I could not validate with any dap4 schema. As a result, I am starting from a dap3.3.xsd schema. I am able to validate a very simple dmr against the schema.