Make handling of CONVENTIONAL variables consistent

[denised]

When multiple solutions have the same CONVENTIONAL alternative, they should use the same values for CONVENTIONAL variables. E.g. all the Energy solutions should use the same values for things like emissions-for-conventional-alternatives.
In the current practice, this is handled as a separate, manual step early in the integration process.

Moving forward, in the python system where it is easier to share things, solutions should be able to declare what their conventional technology is, and that should cause all the conventional values to be loaded from that shared data set.

[denised]

Note: this is already done for the energy solutions: they share conventional VMAs located in the data/energy/vma_data directory. Its nice how the change in vma directory representation makes this much more transparent when looking at the code.