factor configuration into a "modular" multiple-inheritance approach

[scott2b]

Pull config.cpp/h apart such that we have modular configuration structs each with a loadConfig that handles loading it's own specific properties from the SD card config file. For now, just create base structs for RFM95 and for WiFi

The SensorGridPM config implementation should then inherit from these base configs to create an overall config combined from those "modules". PM implmentation will also add it's own parameters. Override loadConfig to load its own parameters as well as to call the load config of the base classes so that all parameters are read into the final struct