Proposed erratum: remove "NaN" from description of POS

[gpdf]

See also ivoa-std/DAP#6 .

There is what appears to be a vestige of the 20150730 PR of SIAv2 in the final spec:

Valid coordinate values are in [0,360] for longitude and [-90,90] for latitude (or NaN).

The use of NaN as an "end of range" flag was in that draft (e.g. "The north pole: POS=RANGE 0 360.0 89.0 NaN") but was replaced by the use of infinities in the final standard. The infinities were subsequently removed by Erratum 1.

The residual presence of NaN just this month caused a Rubin developer to wonder whether it had to be supported. Notwithstanding our more general problem with visibility of errata, could we please process an erratum on SIAv2 to remove the NaN, as well as removing it from the draft of DAP?