Clarify valid content for the "version" field

[lpugin]

The specs do not seem to be clear about what is valid in the version field. They read

The version key MAY be included. The value of the version key MUST be pe2 for incipits that conform to Version 2 of the specification. If a version key is not specified, the incipit SHOULD be interpreted as conforming to Version 1 of the specification.

is "version": "Plaine And Easie 1" valid for v1 incipits?" Or does v2 not allow to specify v1 explicitly at all?

[ahankinson]

Since there is no value defined for version 1, we should probably stick with pe (the value in MARC). But since the empty string also means v1, then we may also have to state that any other value than pe2 must be interpreted as v1, probably with a clarifying note that says that subsequent paec versions are excepted since they will define a new string. (Pe3?)