Add step index

[hampusnasstrom]

There is sometimes a need for step index in a sample deposition. @aalbino2 suggested adding this on the VaporDeposition level but it might make sense to put it already on the SampleDeposition level or even in the base sections on the Process level.
Current normalizers to keep in mind:

• The workflow normalizer in SampleDeposition class has a is_serial() method call which currently only looks at times but should then also look at the step index.
• The time normalizer in the Process class which assumes serial steps: https://gitlab.mpcdf.mpg.de/nomad-lab/nomad-FAIR/-/blob/develop/nomad/datamodel/metainfo/basesections.py?ref_type=heads#L1195