Use range of strings instead of enum for acquisition.technique

[despadam]

#35 adds the field acquisition.technique and an enum with its permitted values. Each subclass of Acquisition gets one of these values with the use of equals_string. While this solves the problem for now, it is restrictive in the sense that people who may want to expand our schema will have to explicitly edit the enum. We should explore: would it be possible to have a range of strings instead?

Originally posted by @despadam in #21