Add optional schema validation for methods that load in XML files

Functionality should be added to validate XML schemas when they are loaded in. This code should be run by the 'load' methods, which will have an optional parameter added to them called 'validate_schema' which will have a default value of false. This code should also be fully unit tested before being used.

Start by locating the XML load methods and the existing unit-test suite; the issue does not name specific files or entry points. Confirm how schemas are currently represented or loaded, then add optional validate_schema behavior with a false default and tests covering validation and the default path.

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