Identifier     get_unit

 Purpose        extract the unit(s) of one or several tags in the data
                part of a given structure (normally AAR)

 Synopsis       unit = get_unit(strucin, tag, stat)

 Arguments      Name      I/O  Type      Description
                -----------------------------------------------------
                strucin    I   struct (AAR)   Sectrum to be checked
                tag        I   string/   name of the tag(s) for which the unit
                               strarr    shall be checked
                stat       O   int       error status 0 if ok, 1 if fatal error

 Returns        unit = lower case strarr containing the value(s) of the TUNIT 
                keyword(s) corresponding to the given tag(s) 
                0 if fatal error occured

 Description    Finds index n of tag in the data part. Reads value of 
                corresponding TUNITn. Returns values.

 Comment        'microns' (LWS) is treated as 'um'

 Example        units = get_unit(saar,'wave',stat)
                units = get_unit(saar,['wave', 'flux', 'stdev'],stat)

 Dependencies   CALLS: get_hdrkey, is_struct, sap_error
                CALLED FROM: arithm, dered (general auxillary routine)

 Category       ISAP

 Filename       get_unit.pro

 Author         E. Sturm (--> ES)

 Version        1.2

 History        0.1   11-10-95  ES  first version
                1.0   30-11-95  ES  V1 delivery
                1.1   29-01-96  ES  new call to is_struct, replace
                                    read_fits_key with get_hdrkey
                      29-02-96  ES  error status added
                1.2   26-07-96  ES  treat 'microns' as 'um'
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Copyright (C) 1995, Max-Planck-Institut fuer extraterrestrische Physik (MPE);
                    Garching, Germany                                       ;
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