Errors in file_io with SQLite

[dlohmeier]

In our changes to the net comparison, we realized that some errors occur when saving and restoring nets with SQLite, which remained undiscovered in the old version. The test is now marked as xfail (pandapower/test/api/test_file_io --> test_sqlite), as it is just a little error in the std_types entry. However, this should be fixed. As a small description: In the sql file IO, the std_types dictionaries are converted to dataframes and then saved in database tables. When restoring them and converting them back to dict, some dtypes are not correct (float to str).