Use exposure time for normalization

[YooSunYoung]

For now, white beam, histogram mode detector data doesn't come with shutter-speed(exposure time).

Per measurement (per file) it doesn't change, but it can vary in different files.

The current normalization method doesn't take it into account.

Update From ECDC
ECDC will add this field as a NXlog and it'll be somewhere under the detector group such as /orca/acquire_time

[YooSunYoung]

@MridulS This is related to the normalization.
I think the exposure time should be exposed as a parameter (from the perspective of sciline) so that user can set it.
It won't be easy to automate finding this parameter in a non-ess-nexus file.

But this issue is about nexus file, so we should be able to find this exposure time in the ess-nexus file.