Use rasters from the file system when calculating environmental suitability

* Remove rasters from the model_run table
* Load raster from file system when calculating environmental suitability
* Ensure that the new NODATA value of -9999 is handled correctly

Performance is better:
* For list of disease occurrences: startup time was 2.8 secs (load raster from DB), now 0.5 secs
* For single disease occurrence: was around 1 sec (caculate in DB), now 0.54 secs