Introduce `lower_bound` on non-CO2 emissions

[jkikstra]

Would it be a good idea to implement a lower bound of zero for all species where no net-negative/removal technologies have been invented yet? This could ensure that reporting issues would be captured sooner.

[danielhuppmann]

Good idea, can you please specify the species (and years?) where that bound should apply?

[jkikstra]

Species: all except CO2 (until someone will submit something where they did actively model a non-CO2 removal technology, and then we change that species)

Years: all