Support: add an `isValid()` method to our CellmlFile, SedmlFile, and CombineArchive classes

[agarny]

This method is to be used when we actually want to make use of a CellML file, SED-ML file, or COMBINE archive. Things to keep in mind:

• See CellmlFile::isValid(), SedmlFile::isValid(), and CombineArchive::isValid() in OpenCOR.
• Check whether we will need to create a parent class (StandardFile) for common methods and properties.
• Will need to enhance our Issue class to account for various types needed by SED-ML files and COMBINE archives.
• A CellML file needs to be flattened before being validated.