overwrite duplicate diagnostics

[juliasloan25]

Purpose

Instead of raising an error when a user tries to add a diagnostic that already exists, this PR warns the user and overwrites the existing diagnostic with the new entry.

closes #2709

[Sbozzolo]

Maybe print out more information on what was there before overwriting?

[juliasloan25]

Maybe print out more information on what was there before overwriting?

Do you want me to also show what it's getting overwritten with?

[Sbozzolo]

Maybe print out more information on what was there before overwriting?

Do you want me to also show what it's getting overwritten with?

No, we assume that users will know what they called.

[juliasloan25]

Okay! I updated the warning message so it will print something like

┌ Warning: overwriting diagnostic `mse` entry containing fields
│ [mse, Moist static energy, moist_static_energy, J/kg, Moist static energy]
└ @ ClimaAtmos.Diagnostics ~/.julia/packages/ClimaAtmos/gIAbX/src/diagnostics/diagnostic.jl:155