Make `NWBFile.identifier` resetable before it is written

[rly]

From @yarikoptic via slack

When constructing an NWBFile, identifier is required (but see #954 which suggests making it optional), and once set, it cannot be changed. This means the following workflow will not work:

create file instance, populate all data relevant, pass that instance to dandi's organize to figure out the file name given the data, assign that as identifier to the instance

@yarikoptic would like to be able to construct an NWBFile with a dummy identifier and update it later before writing the NWBFile.

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