Book driven updates

[t-kalinowski]

• fix super()$initialize() in a %py_class% (previously only R6 style usage like super$initialize(), with an uncalled super, would resolve correctly)
• make it easier to use register_keras_serializable() with a bare R function
• register S3 methods for t(), aperm() and all.equal() for backend tensors.
• make dpi globally configurable in the plot(model) method.
• (re)export py_help(), named_list(), joining recently added py_require(), import(), py_to_r(), r_to_py()
• expand support for convert=FALSE in builtin dataset loaders like dataset_mnist().