Make slider available for all years if it is set in a single year

[mabijkerk]

If a transition path is created by stringing ETEngine scenario IDs, it does not have to be the case that all scenarios contain the same inputs. If one scenario does not have a certain input, while the other does, the following happens:

For 2030 the value of the housing stock sliders is blue/available and can be clicked directly. For 2040 and 2050 it is grey/unavailable.

This seems to be an explicit design choice. One of our clients asked us to change it. The suggested change is that a slider becomes available for all scenarios if it has been set for at least one scenario in the transition path. @ChaelKruip I don't see an issue with this suggested enhancement. Do you agree?

@noracato this issue is an enhancement at the moment, so there is no explicit priority.