syntax: writegdx ('gdxFileName'
                  ,'dataType','gdxSymName',dataMatrix
                 [,'dataType','gdxSymName',dataMatrix][...]
                 (optional) UELList)

Write Matlab data in dataMatrix to GDX file gdxFileName as symbol
gdxSymName. The dataType of gdxSymName must be either 'set' or
'parameter', although any abbreviation of these is also accepted.
UELList is an optional cell of strings.  The gdx file will use it as
its UEL (unique elements) list, otherwise, the default UEL list 
'1','2','3',... will be used.
Note that writegdx does not merge the new data with existing data in
gdxFileName: if the file exists, it will be overwritten.

Note: this program is modeled to have unlimited inputs, so that GDX
files containing multiple symbols can be created.  For example:

   writegdx('tang.gdx',  'set','foo',A, ...
                         'parameter','bar','B')

creates a GDX file tang.gdx containing two symbols - the set foo and
the parameter bar.

The dataType given determines how the dataMatrix is interpreted.
Assume size(dataMatrix) = [m,n].
if dataType is 'set',
  the output gdxSym will have dimension n and cardinality m, where
  each row of dataMatrix contains the indices for an element in gdxSym
if gdxSymName is a parameter,
  the output gdxSym will have dimension n-1 and cardinality m, where
  each row of dataMatrix contains the indices for an element in gdxSym
  in columns [1 .. n-1] and the element's value in column n.