NAME:
  parsekey
 PURPOSE:
  Extract FITS header information by parsing supplied keys
 DESCRIPTION:

  Note: the DATE-OBS keyword often now holds the date and the time both, joined
    by a T between the two fields, example: 2013-08-05T23:27:15.  You can
    associate both DATE and UT with this keyword and DATE will only take
    what preceeds the T and UT will take what follows the T.

 CATEGORY:
  CCD data processing
 CALLING SEQUENCE:
  parsekey,hdr,keylist,info
 INPUTS:
  hdr     - String array containing the FITS header to "read"
  keylist - 3,n element string array.  For each key, there are three elements:
                0 - "standard" key name
                1 - item descriptor, K - keyword, T - template, V - value,
                                     W - keyword but just take first word.
                2 - information (contents match what descriptor indicates)
 OPTIONAL INPUT PARAMETERS:

 KEYWORD INPUT PARAMETERS:
  UNKNOWN - This is a two-element string array.  The first element is the
             "name" of a filter to look for.  If this name is found then
             it is replaced by the string in the second element.  This is
             used to rename a filter that is tagged in the header as unknown.
             The default is ['',''] which does nothing.

 OUTPUTS:
  info    - Anonymous structure containing information gleaned from header.
             tags:
                 airmass - float value for airmass as read from header
                 jd      - Double, julian date of middle of exposure
                 date    - String containing the UT date.
                 ut      - String containing the UT start time of the image.
                 expdelta - Time, in seconds between successive frames if
                               the image is 3-d, not relevant otherwise
                 exptime  - Exposure time in seconds.
                 imfile   - Original filename of image.
                 filter   - String that identifies the filter used.
                                If you use a keyword that grabs more than
                                one position (eg., FILT_*) then this field
                                will contain information about all the filters
                                in wheel order where the filter strings are
                                separated by '+'.
                 object   - String that identifies the object imaged.
                 ra       - Right ascension of object (radians) (< 0 if invalid).
                 dec      - Declination of object (radians) ( < -100 if invalid).
 KEYWORD OUTPUT PARAMETERS:

 COMMON BLOCKS:

 SIDE EFFECTS:

 RESTRICTIONS:

 PROCEDURE:

 MODIFICATION HISTORY:
  1997/09/05, Written by Marc W. Buie, Lowell Observatory
  2000/02/05, MWB, added kludge fix for new DATA-OBS "Y2K" format.
  2002/09/03, MWB, changed Str_sep call to strsplit
  2006/07/13, MWB, changed to support multiple filter wheel instruments
  2006/08/09, MWB, changed to strip path from file name if found.
  2011/01/25, MWB, added UNKNOWN keyword
  2013/08/05, MWB, added fix to allow getting time from DATE-OBS keyword