Consequential system model

[cmutel]

This should not be so difficult. The consequential system model should do the following:

• Change the way that markets are filled up, and use technology levels instead of production volumes. Production volumes are used for splitting activities with the same technology level.
• Byproducts can be ignored. They are outputs for which some other activity provides a consumption or production process. We could flip these to negative inputs, but this isn't necessary and doesn't accomplish anything, and would require code and tests, so we leave it for now.
• Constrained markets will need links to the alternative production activities, and a reduction in the amount of the activity used in step one.

[cmutel]

Some good progress on the consequential branch. However, constrained markets are tougher than expected, see many miles of pain... need to change other datasets based on activity links in constrained exchanges, etc. etc.

[cmutel]

Preliminary version add in #133.