use sim_mode only in init

[juliasloan25]

Purpose

Remove all uses of sim_mode after initialization, which was mostly in postprocessing/diagnostics. Note that now all available diagnostics will be plotted at the end of the simulation, where before we specified a subset to plot based on the simulation type.

closes #1138

To-do

• check CI output diagnostics

Content

• shared postprocessing for slabplanet and AMIP simulation types
• rename amip_diags_handler -> diags_handler
• delete plot_diagnostics config option (redundant with use_coupler_diagnostics)
• remove mode_specifics from CoupledSimulation altogether
• rename amip_diagnostics_setup to coupler_diagnostics_setup

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