Remove ffig internals from flask API

[petr-tik]

Currently copying too much ffig.main function into the request handler. As soon as ffig exposes an endpoint, use that and remove calls to main ffig

ffig_subfolder = '/home/ffig/ffig/'
template_dir = os.path.join(ffig_subfolder, 'templates')

env = ffig.FFIG.set_template_env(template_dir)

template = env.get_template("py3.tmpl")

m = ffig.FFIG.build_model_from_source(
    "filename.hpp", mod_name, [("filename.hpp", inp_file)])
classes = m.classes
api_classes = ffig.FFIG.collect_api_and_obj_classes(classes, 'FFIG:EXPORT')
output_string = ffig.generators.render_api_and_obj_classes(mod_name, api_classes, template)