New Variable Statistics: Prices|Final Energy|Gas mix

[maxnutz]

Goal:

Add a new function to extract the statistics of variable Prices|Final Energy|Gas mix
Price of Final Energy|Gas mix per unit. This includes Gas from various sources including Fossil and Non-Fossil Gas.

To Dos:

• Create a new branch linked to this issue
• Write your pypsa-statistics and add it as a separate function to statistics_functions.py (please note the naming and structural conventions!)
• add a comprehensive docstring to your function
• add the mapping variable_name <> function_name to mapping.default.yaml (and your personal mapping-file)
• Add a testing routine for your Function to tests/ - stick to the testing-README
• make sure, that the newest version of main is merged into your feature Branch
• open a pull request and assign @maxnutz as reviewer