Replace Data_Investment_new.xlsx with official reference data set

Now the file Data_Investment_new.xlsx is used to specify inputs for grid reinforcement candidates (electrical parameters and costs). This works but is somewhat redundant and "clumsy" since that Excel file has been generated partly based on manual Excel copy-pasting, partly based on Excel formulas, and partly based on Python scripts. Ideally (at some point), all code should retrieve data about the reference system directly from the "official" data set. Advantages is that it ensures consistency and makes it more easily traceable where data is coming from.