Add utility processing algorithms

[nmaarnio]

We could add the following functionalities as processing algorithms:

• Split input multiband raster into singleband rasters
• Combine rasters (singleband and multiband) into one. Requires that all rasters have same grid properties
• Harmonize nodata. This would take multiple rasters and convert all of them to use same value as their nodata. For example, raster 1 uses -9999 as it's nodata and raster 2 uses -99999 as it's nodata. The user wants all rasters to use -9999, so all -99999 values in raster are converted to -9999 and the metadata updated
• Overwrite nodata. Not entirely sure if this is wanted/needed, but sometimes the metadata is just set wrong (metadata says -9999 but some other value actually represents nodata). This would simply update the metadata to the number user specified.

[nmaarnio]

EIS Toolkit PR GispoCoding/eis_toolkit#371 adds a few utilities that could be added as processing algorithms