[ISA-XLSX] Unify File Name and Identifier for assays/studies

[Freymaurer]

Currently assays and studies are handled differently regarding the two fields Identifier and File Name.

The Problem

• There is no Identifier in assays.
• The File Name is always set to isa.study.xlsx/isa.assay.xlsx.

Proposed Solution

• Add a Identifier field to assays.
• Remove File Name from both assays/studies.

Alternative Solution

• Remove both Identifier and File Name fields as these are handled by parent directory name.
• This would be the solution with the least amount of information duplication but would set ISA-XLSX in a very specific ARC setting. I am unsure if this a no-go or to-go.

@HLWeil @muehlhaus @omaus @kMutagene