Streamline version string of CLI between emulators (at least FaIR and MAGICC)

[jkikstra]

At the moment it looks like we need to do:

--model "magicc" --version "v7.5.3"

and

--model "fair" --version "1.6.2"

It's confusing to have "v" for magicc but not for fair.

---

P.S. note that CICERO has a different version naming entirely: "v2019vCH4"

Line for FAIR:

climate-assessment/src/climate_assessment/climate/fair.py

Line 8 in 315a465

DEFAULT_FAIR_VERSION = "1.6.2"

Line for MAGICC:

climate-assessment/src/climate_assessment/climate/magicc7.py

Line 8 in 315a465

DEFAULT_MAGICC_VERSION = "v7.5.3"

Line for CICERO:

climate-assessment/src/climate_assessment/climate/ciceroscm.py

Line 7 in 315a465

DEFAULT_CICEROSCM_VERSION = "v2019vCH4"

(ping @phackstock)

[phackstock]

Thanks @jkikstra, that'll prove really useful in the future.

[jkikstra]

Just an extra note: I think these "DEFAULT_*_VERSION" variables are actually not used in the output data (and so do also not feature in the test-data files), which would make life easier in changing this.
One suggestion would we to change all three to "ipcc_ar6" or "ipcc_ar6wg3" or simply "ar6".