Uniform timestamps

[j-brendel]

Please adjust timestamps in input and output files to a uniform standard:

• column name: time (or arrival time etc.) - not date, not datetime etc.
• format: YYYY-MM-DD HH:MM:SS

Places to be changed (list may not be complete):

• electrified stations: energy_feed_in, external_load -> start time
• example files: example_external_load.csv; example_pv_feedin.csv;
• output files: gc_power_overview_timeseries.csv; rotation_summary.csv; vehicle_socs.csv

[j-brendel]

solved by #189