Make units required for all schemas

Likely it is beneficial to make all unit fields required in all schemas, since it would be hard to interpret the values correctly otherwise.

This may be already the case for some of the schemas (SEM schema for example), but is certainly not true in all cases

Example: https://github.com/kit-data-manager/Metadata-Schemas-for-Materials-Science/blob/219df3e7e245be694488aecd3813047af488aca1/SEM-FIB_Tomography/SEM_FIB_Tomography_acquisition_Image_Schema.json#L193

It may also be beneficial to have a more concise concept of units instead of simple enums, because it would help to derive from the schema if something is a temperature unit, a measurement unit and so on. May also reduce duplicate definitions. But I guess that would be an advanced topic in comparison to the initial topic of this issue.

Start by reviewing the schema files and the SEM-FIB_Tomography/SEM_FIB_Tomography_acquisition_Image_Schema.json example around line 193, comparing how unit fields are currently declared. Done means unit fields are required consistently across all schemas; the issue also identifies a separate, more advanced question about representing units concisely.

Written by the indexing model from the issue text.