start_iso8601 and end_iso8601 not validated

[vedran-kasalica]

The ISO_start and ISO_end times are obligatory, however, the library does not check whether the fields are valid.

For example, here the start is in 2022, while the end is in 2019.

This could cause issues when doing post-processing, or implementing features that utilise the time element, e.g., random access.