Deprecate passing file_tuningmodel_x flags

[znicholls]

These flags allow the user to specify other files from which pymagicc should read config. They are hugely confusing to use and can cause unexpected behaviour. For our sanity, we should prevent users from using them, instead providing a convenience function load_tuningmodel_cfg which can take a list of tuningmodels and merge them into a config which can then be passed to MAGICC.run. This gets the config blending into Python, where it's simple to debug, and out of the calls to ./magicc, where it's much harder to debug.

@lewisjared or @znicholls will have a go unless someone else gets there first.