Add some higher level helper methods

[jonc125]

For instance

• a function std::unique_ptr<neuroml2::NeuroMLDocument> neuroml::parseFile(const std::string& path_to_file) for one-liner validate & parse to object
• a method NeuroMLDocument::writeToFile(const std::string& path_to_file, ios_base::openmode mode = ios_base::out) for one-liner file output