Sample manifest importer should allow unmapped amount column

[silby]

The amount key must be mapped to a column in the SampleManifest importer. I want to be able to leave this column unmapped, to import lists of samples without tracking the aliquots.

The importer should become idempotent when amount is unmapped: if multiple rows identify indistinguishable samples, only one sample should be added. If the amount column is mapped, a row with an empty amount should probably abort the import.