syntax: [x, uels] = readgdx ('gdxFileName', 'gdxSymName')

Read the coordinate/index information from the GDX symbol
'gdxSymName' into Matlab.  The output is a matrix that contains the
data of the set or parameter 'gdxSymName'.
'gdxSymName' can read a set or parameter of any valid dimension.

Assume gdxSymName has dimension d and cardinality C
if gdxSymName is a set,
  the output x will have size(x) = [C,d]
  where each row of x contains the indices for an element in gdxSymName
if gdxSymName is a parameter,
  the output x will have size(x) = [C,d+1]
  where each row of x contains the indices for an element in gdxSymName
  with the (d+1)'st column being the value for that element
The optional output uels returns the UEL list of the gdx file