TLDR

Add the ability to model multiple emission types at the same time.

The commits

Enable modeling multiple emission types

Add the ability to model multiple emission types at the same time.

This feature is composed by various parts:

1. Need to be able to define in sets what are the different emission types that need to be modeled
2. Need to change the emission related parameters (i.e. emission per tech, emission taxes, emission caps) to be able to collect parameters for all the emission types defined
3. Need to consider all defined emission types' costs and constraints in the model optimization
4. Need to change the outputs in order accomodate for a variable number of tracked constraints

Add emission tests
Add a unit test to make sure that the emission functionality is working as intended.

I tested how hypatia models emissions by checking that:
1. The model correctly tracks the emission generate by the model solution
2. The model correctly considers emission cost to find the optimal solution
3. The model correctly considers emission caps (global and regional) to find the optimal solution

We check these feature in the 4 main situations:
1. Single region, planning
2. Single region, operation
3. Multi region, planning
4. Multi region, operation

The tests are run by
python3 -m unittest hypatia/backend/tests/EmissionTests.py