[BUG] Assay identifier is not added to isa.investigation file

[andreaschrader]

Describe the bug
When registering an assay, there is no assay identifier row in all three isa.xlsx files.

To Reproduce
Initiate an ARC, add a study with an assay (arc a add -s MyStudy -a MyAssay)

Expected behavior
A row with the assay identifier similar to study identifier.

OS and framework information (please complete the following information):
macOS
arc commander v0.5.0 same for v1.0.0 (preview2)

[HLWeil]

Hey @andreaschrader,

this is because this field does neither exist in ISA Tab nor ISA Json (which is a shame). Having the Identifier as a argument is just for user convenience.

Feel free to add your perspective in nfdi4plants/ARC-specification#74.