[wktraster] Add a HasNoDataValue function

[pracine]

We need a ST_HasNoDataValue() function in order to determine if the stored NoData value is significant or not because it is not possible to store a null value (is it?)

[mloskot]

Any idea how are you going to implement it?

Are you going to use 4 spare bits of pixel type (see ​RFC1-SerializedFormat ?

[pracine]

Only one. Is it possible to store a null value in the header using gdal2wktraster.py?

[mloskot]

Pierre, do you mean header of the raster?

Yes, should be as we store various properties in header already (see wkblify_raster_header​ function).

[dzwarg]

I created a patch that adds:

• rt_band_get_hasnodata_flag
• rt_band_set_hasnodata_flag

To the c api. These have unit tests in the test/core folder, and I updated the RFC to note that the 2nd highest bit of the 'data' byte is occupied by the 'hasnodata' bit.

[dzwarg]

Implementation of hasnodata flag in the c api.

[pracine]

The function is implemented at the core and the PostGIS levels but gdal2wktraster.py remains to be modified to set the flag when it is needed.

[pracine]

Modified gdal2wktraster.py so now it sets the HasNodataValue flag in the WKB.