Fix a failing test due to GMT API change (#556)

The test `test_put_matrix_grid` fails due to recent changes of GMT API
in GenericMappingTools/gmt#3848.

Before this PR, for a matrix grid, calling GMT_Write_Data always writes
the matrix grid as a matrix, no matter the geometry is `GMT_IS_POINT` or
`GMT_IS_SURFACE`.

This PR declares it as a bug (or an improvement). With that PR merged,
`geometry=GMT_IS_POINT` writes a matrix, `geometry=GMT_IS_SURFACE`
writes a the matrix grid as a netCDF file.

This test expects a matrix, but the new API writes a netCDF grid.
That's why it fails.

This PR fixes the issue by simply changing `GMT_IS_SURFACE` to
`GMT_IS_POINT`, so that the test can pass for GMT 6.1.0, 6.1 and master
branches.

Author: seisman

pygmt/tests/test_clib_put_matrix.py

@@ -76,7 +76,7 @@ def test_put_matrix_grid():
             with GMTTempFile() as tmp_file:
                 lib.write_data(
                     "GMT_IS_MATRIX",
-                    "GMT_IS_SURFACE",
+                    "GMT_IS_POINT",
                     "GMT_CONTAINER_AND_DATA",
                     wesn,
                     tmp_file.name,