MGH_UNPACK Name
  
   MGH_UNPACK Purpose
  
   This function reproduces (some of) the functionality of the Fortran
  
   90 UNPACK procedure, ie it reverses the packing of data according
      to the value of a mask variable. See also MGH_PACK.
Calling Sequence
  
   Result = MGH_UNPACK(Vector, Mask) Inputs
     Vector:     A 1-D array of values to be unpacked. Number of elements
              
               must be >= number of 1s in Mask. 
  
   Mask:       An array of 1s and 0s defining the locations where
              
               the elements of Vector will be unpacked.
Keyword Parameters
  
   MISSING_VALUE: Set this keyword to a scalar to specify the value
              
               that will be used for missing locations, i.e. those where
              
               the mask value is zero. Outputs
 
   The function returns an array with the same type as Vector and the same
      shape as Mask. 
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    This software is provided subject to the following conditions:
    1.  NIWA makes no representations or warranties regarding the
    
     accuracy of the software, the use to which the software may
    
     be put or the results to be obtained from the use of the
    
     software.  Accordingly NIWA accepts no liability for any loss
    
     or damage (whether direct of indirect) incurred by any person
          through the use of or reliance on the software.
    2.  NIWA is to be acknowledged as the original author of the
    
     software where the software is used or presented in any form. 
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Modification History
  
   Mark Hadfield, August 1996: 
              Written.
      Mark Hadfield, May 2000: 
      
       Updated for IDL2 syntax and newer data types.