[Geotiff] No proj4 definition for CH1903/LV03 (EPSG 21781) maps

[Christian Aichinger]

Hi,

I have maps with CH1903/LV03/EPSG 21781 coordinates. They worked fine 
with previous versions of libgeotiff. The current libgeotiff can still 
read them, but can't produce a proj4 definition for them. Unfortunately, 
I needed that feature.

The problem seems to be the May 2012 Hotine rework[1]. EPSG 21781 now 
gets mapped to CTProjection == CT_HotineObliqueMercatorAzimuthCenter, 
but that is not supported in GTIFGetProj4Defn() in geotiff_proj4.c, only 
CT_ObliqueMercator is.

I'm not sure how to best approach this problem. Should projop_wparam.csv 
be modified so it gives CT_ObliqueMercator with the correct parameters? 
Or should CT_HotineObliqueMercatorAzimuthCenter just be handled in 
GTIFGetProj4Defn()?

I've attached a sample file that shows the problem. To reproduce, run 
`listgeo -proj4 CH1903-sample.tif` and observe that no proj4 definition 
is shown.

Cheers,
Chris

[1]: http://lists.maptools.org/pipermail/geotiff/2012-May/000720.html

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$ listgeo -proj4 CH1903-sample.tif
Geotiff_Information:
   Version: 1
   Key_Revision: 1.0
   Tagged_Information:
      ModelTiepointTag (2,3):
         0                 0                 0
         620000            172000            0
      ModelPixelScaleTag (1,3):
         2.5               2.5               0
      End_Of_Tags.
   Keyed_Information:
      GTModelTypeGeoKey (Short,1): ModelTypeProjected
      GTRasterTypeGeoKey (Short,1): RasterPixelIsArea
      ProjectedCSTypeGeoKey (Short,1): Unknown-21781
      ProjLinearUnitsGeoKey (Short,1): Linear_Meter
      End_Of_Keys.
   End_Of_Geotiff.

PCS = 21781 (name unknown)
Projection Linear Units: 9001/metre (1.000000m)

PROJ.4 Definition:

Corner Coordinates:
Upper Left    (  620000.000,  172000.000)
Lower Left    (  620000.000,  171875.000)
Upper Right   (  620125.000,  172000.000)
Lower Right   (  620125.000,  171875.000)
Center        (  620062.500,  171937.500)