Remove NaN from POS description

[gpdf]

SIAv2 contains the following throwaway reference implying that a NaN might be acceptable in a POS query parameter:

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

@timj pointed this out in the course of implementing an SIAv2 interface associated with the Rubin Observatory "Data Butler".

There is no other reference to NaN in the standard, and it's in conflict with language a few sentences above that suggests the use of infinities as special values (language which has already been suppressed by SIAv2 Erratum 1).

It seems to very likely be an editing oversight, a vestige of the 20150730 PR of SIAv2, in which NaN was used in RANGE where the final standard uses infinities.

Can we a) remove this vestige from DAP and b) issue an erratum on SIAv2?