Provide a way to remove coordinates from some fibres in a pfsConfig/Design

In the discussion of PFS ops, Masayuki Tanaka points out that for open use programs which share fibres we need to have a way of distributing pfsConfig files that omit the positional information for some fibres.
Please make this possible. Some options that come to mind are:

1. simply omit some fibres from the pfsConfig
2. set the objId to 0x0 and the pfiCenter/pfsNominal to NaN
3. define a new fiberStatus IGNORE and set the pfiCenter/pfsNominal to NaN

I don't like the first as it means that the DRP doesn't know which fibres are illuminated. The other two are viable; I prefer the special fiberStatus