create minimal set of variable-functions for final energy

[maxnutz]

Goal

Finalize setup by creating example functions for pypsa network-evaluation

What to do

Variables

Final Energy [by Carrier]|Electricity	Energy consumption of electricity excluding transmission/distribution losses

Final Energy [by Sector]|Transportation	Energy consumption by the transportation sector excluding transmission/distribution losses

• create one function per variables for the 2 variables defined above, named like the variable name. Replace "|" with "__" and " " with "_" and delete "[" and "]".
• read out the value of the variable from the pypsa-NetworkCollection in the corresponding function defined
• embedd the code into the existing structure

To Dos:

• create the statistics-function in statistics_functions.py
• add the mapping in mapping.default.yaml for the variable name mapping on the function
• write a numpy-style docstring for the statistics-function
• integrate a dummy pypsa.statistics - statement to test the workflow.
• test the workflow and adapt, if adaptations are necessary for the workflow to pass

To Dos for the user bevor the review (to not tackle this ones!)

• integrate the pypsa-statistics-statements to read out the pypsa the statistics from NetworkCollection
• implement the unit-conversion in method structure_pyam_from_pandas() in Network_Processor.
• create merge request and review changes.

Additional Convention:

• work in the development-branch connected to this issue
• commit your changes to this development-branch
• do NOT yet create a merge request when you are ready

[maxnutz]

finised with #6