Support writing high-precision timestamps to hdf5

[maxnoe]

Please describe the use case that requires this feature.

Currently, we write out all timestamps as TAI MJD.
That is one of the possible formats for the CTA "low precision" timestamp.
However, some things might still require the high precision timestamps, so we should have an option on the datawriter whether to use high or low precision timestamps for event times and similar things that are high precision in the input.

Describe the solution you'd like

Add option to store timestamps that are high precision keeping this high precision.