- Remove vrt_time_calendar() in favour of vrt_time_fields()
- Rename VRT_ERR_INTEGER_SECONDS_MISMATCH to VRT_ERR_MISMATCH_INTEGER_SECONDS for consistency
- Remove vrt_time_difference() in favour of vrt_time_difference_fields()
- Fix consistency error. Sometimes it's integer_seconds_timestamp and sometimes integer_second_timestamp
- Make validate parameter in read/write an enum