input PARAMETERS with limited list of values : better description

[Bonnarel]

Several SIAP2.0 parameters have a limited list of possible values
– Some have lists limited by protocol (and obscore)

• POL (Stokes, LINEAR, etc..)
• DPTYPE (image, cube, visibility, timeseries ;..)
• CALIB : levels
• FORMAT : fts, jpeg , png, etc..

– Some have free string values

• COLLECTION (HST, WISE, etc...),
• FACILITY (VLT, Keck, Chandra),
• INSTRUMENT (ACS, MEGACAM, etc.)

– PARAMETERs less useful if we have no prior idea of their possible
values.
It often happens that services do not provide these lists of parameters although section 2.1.1
However Section 2.1.20 of SIAP2.0 states :
Service PARAMETER self description
Any service may include a DataLink service descriptor in the vOTable output to describe itself. this descriptor would describe the supported query parameters (standard and custom) including list of values for those with a fixed list (eg. COLLECTION INSTRUMENT FACILITY DPTYPE CALIB and FORMAT) *

This will allow to discover "possibilities" of the service prior to usage and optimize the queries

The "MAY" is obviously too weak. Version 2.1 could state SHOULD or MUST

[pdowler]

I'm not sure changing from may to should would really accomplish anything.

Changing this to MUST isn't compat enough for SIA-2.1, but it could pass muster for SDA-2.1 (#10)... let's see what Markus says about when to use MUST in ops.