Add .config attribute to MAGICC

[znicholls]

Is your feature request related to a problem? Please describe.

The config that is used to MAGICC currently exhibits hysteresis i.e. the config used in a given run depends on what happened to the files on disk in previous runs. This can make results change unexpectedly and debugging extremely difficult.

Describe the solution you'd like

Add a .config attribute to MAGICC. On instantiation, this attribute will be filled with MAGCFG_DEFAULTALL.CFG, overwritten by MAGCFG_USER.CFG, so the user can easily see what their starting config is. On calls to update_config and set_config, this attribute will be updated. The config would only be written to MAGTUNE_PYMAGICC.CFG as part of a call to MAGICC.run, which makes it easy to know what config will be used and also means we only have to write to disk once, no matter how many update_config and set_config calls are made in the meantime.

Describe alternatives you've considered

None have sprung to mind but there may be a better way.

Additional context

This came up as @lewisjared and I were doing runs where the results kept changing in weird ways. We eventually figured out the bug and that this would be a solution.

@lewisjared or @znicholls will have a go at this if no-one else gets there first.