S1S2 target coverage target_validation.py

[max-tikehaucapital]

S1S2 targets are supposed to have coverage_s1=coverage_s2 (otherwise coverage_s2 is not required since only coverage_s1 is used to adjust the reduction_ambition). For example it is checked when converting a S1, S2, or S1S2S3 target into a S1S2 one. However it is not done when the target has scope S1S2, therefore I suggest to fix coverage_s1 and coverage_s2 to the weighted average of both previous coverages based on base year ghg emissions (this is what is done to convert targets into a S1S2 target). To do this I added a variable s1s2 in the validate method and create a method which implement this modification.