Add explicit names for xcu.temps and xcu.poweredup keyword fields.

Adding FITS cards fields to the XCU keywords in INSTRM-427 caused the internal names to change from "val1_N" to "valN" (because the fields have to be defined individually instead of as a list of 12 floats). This caused problems when the archiver was restarted at LAM.

To maintain backward compatibility, the new fields should be explicitly assigned "val1_N" names.

[ That said, the next Issue will ask whether we should assign "real" names instead (later). ]