Add warning when preindustrial CO2, CH4, and N2O are changed from default values

[kdorheim]

As part of the update to using the AR6 RF equations, preindustrial CO2, CH4, and N2O should be treated as fixed parameters since in this PR I added log-level warnings for when these values are changed from their respective default values.

See here

> library(magrittr)
> here::here("inst", "input", "hector_ssp119.ini") -> ini
> core <- newcore(ini, suppresslogging = FALSE, loglevel = LL_WARNING())
> run(core)

Compared with

> setvar(core, dates = NA, var = PREINDUSTRIAL_N2O(), values = 100, unit = getunits(PREINDUSTRIAL_N2O()))
> reset(core)
Hector core:    Unnamed Hector core
Start date: 1745
End date:   2300
Current date:   1745
Input file: /Users/dorh012/Documents/Hector-Versions/v3/hector/inst/input/hector_ssp119.ini
> run(core)

now returns

[kdorheim]

Is this enough warning? we also have warnings in the ini files, what do you think @bpbond

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[kdorheim]

re the testing in R... hmmm I don't know we don't really do any other log testing in R so other than "do the logs exist" so I am inclined to say no what do you think?

[bpbond]

Makes sense. Thanks.

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