`filewriter`: selog `ICP_DAE_TD` and `ICP_SYS_TD`

[Tom-Willemsen]

The existing .nxs files always contain these datasets, which describe time differences between the DAE and computer clocks.

These differences do not exist in the same way in the streaming system. Therefore we need to decide what we do with these datasets.

Potential differences from existing files

I am proposing to not write these datasets, as their meaning is sufficiently different in the streaming system that these names are confusing. If downstream consumers are using these datasets, I would like to understand why, as the assumptions may have been broken in streaming system.

[Tom-Willemsen]

@martyngigg reports that from a Mantid perspective "The fields in #90 are not used at all"

[martyngigg]

The only slight issue is that since they are SE logs then they are simply loaded as is in the file and any script in the wild could, in theory, be accessing them without our knowledge. For HPRD though, as they use scripts shipped with mantid, they are definitely not used there.