Added source identifer for guide stars and related changes.

Added a source identifier for each guide star for provenance.
With this, also included a CATID keyword so that the
corresponding catalogue can be traced.

Added observation time to the GUIDESTAR HDU so that
the star coordinates can be corrected for parallax.

Corrected proper motion entry so that it is specified
for both RA and DEC directions.

datamodel.txt

@@ -360,32 +360,41 @@ permitted values is limited to a set to be specified by the DRP team.
 There will be a mechanism for adding to this set.
 
 The GUIDESTARS table lists:
+      sourceId       64-bit int
       ra             64-bit float (degrees)
       dec            64-bit float (degrees)
-      pm             32-bit float (deg/s)
+      pm_ra          32-bit float (deg/s)
+      pm_dec         32-bit float (deg/s)
       parallax       32-bit float (degrees)
       flux           32-bit float (nJy)
       color          32-bit float
-      AgId           32-bit int
-      AgX            32-bit float (pixels)
-      AgY            32-bit float (pixels)
+      agId           32-bit int
+      agX            32-bit float (pixels)
+      agY            32-bit float (pixels)
 
 Where the:
+    * sourceId corresponds to the unique source identifer for the guide star
     * ra corresponds to the Right Ascension of the guide star
     * dec corresponds to the Declination of the guide star
-    * pm is the proper motion of the guide star
+    * pm_ra is the proper motion of the guide star along the RA direction 
+    * pm_dec is the proper motion of the guide star along the DEC direction
     * parallax is the observed parallax of the guide star
     * flux is the total flux for the guide star
     * color is the B-V color for that guide star
-    * AgId is the identifier for the AG camera that will observe that guide star
-    * AgX is the X-position on the corresponding AG camera where the guide star
+    * agId is the identifier for the AG camera that will observe that guide star
+    * agX is the X-position on the corresponding AG camera where the guide star
           is expected to be detected
-    * AgY is the Y-position on the corresponding AG camera where the guide star
+    * agY is the Y-position on the corresponding AG camera where the guide star
           is expected to be detected
 
 Additional keywords are required for the GUIDESTARS HDU:
-    * EPOCH epoch for the celestial coordinate system used to measure ra and dec.
+    * OBSERVATION_TIME the time at which the guide star coordinates were measured.
+        This should be accurate to the order of weeks to ensure that the
+        parallax can be incorporated into the measurements.
     * TELELEV the telescope elevation (degrees)
+    * CATID the identifier of the catalogue containing the guide stars.
+        This implies that all the guide stars in this table originate from 
+        a single catalogue.
 
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