feat: verify type of event_index has 8 bytes

[jokasimr]

event_index references positions in the event_id and event_time_offset arrays.
It's dtype has to be wide enough to contain integers that are the size of the event list.

We expect that some instrument might see more than 2^32=4B events during one experiment, therefore event_index must be 8 bytes.

[jl-wynen]

Did you get feedback from ECDC about this?
The index can safely be 4 bytes if there are less than INT_MAX events in the file. So I think the test should only require 8 bytes if that is strictly needed in that particular file.

[jokasimr]

But the type has to be decided when the file is initiated, and at that time the filewriter has no idea about how many events will eventually end up in the file.

In any case, the event_index dataset will never be a lot of data. Per day it will accumulate 8 * 14 * 3600 * 24 bytes per event mode detector, or 10MB, which is negligible.