application - NXtomo: add 'half_acquisition' field.

[payno]

This PR propose to add the "half_acquisition" field to the NXtomo application.

• Half-acquisition scanning mode allows the field of view to be increased. Processing needs to be aware of it.
• The name half-acquisition is commonly accepted amount the community.

Note:
In a perfect world, the NXtomo could store pre-processed sinograms / projections directly, and this field would not be needed.
In reality, however, NXtomo applications are generated from detector raw data, with the processing embedded in the processing pipeline so that computation can happen on the fly. This makes the process less memory-intensive and this field required.