add io functions for various csv files

All files should be read from and written to in the same way: inside blabpy but more importantly when using blabpy. Let's not assume that `pd.read_csv`/`pd.to_csv` will do the right job. This already happens for cha/opf files but I should add functions for various csv files: exportted from cha/opf, sparse_code csvs, all_basiclevel.