[Proj] Use the GTX null value for vertical shift exclusion.

[paul.chavent at fnac.net]

Hi.

Vertical grid shift values specified with GTX files are  rejected outside of the [-1000;1000] range. I don't know if it is the  expected behavior since the GTX files allow any values excepted the null  value.

I've opened a ticket (http://trac.osgeo.org/proj/ticket/271) to address this issue. I've submitted a patch that use the 88.88880 null value for rejecting bad values.

Perhaps a better fix would be to add a null_value (or range) field to the CTABLE and check against this value (or range).

Regards.

Paul.