Implement Polygon per Cell functionality

[mvmaltitz]

We need a function and unit test that counts the polygons in a grid cell and assigns a value from 0 to 5.

Signature:

raster_from_score(bbox, country_boundary, cellsize, output_path, crs, input_polygons)

Inputs:

bbox - the grid aligned bounding box for the analysis area. e.g. if the grid size is 100m the length and width will be exactly divisible by 100m
country_boundary - a QgsVectorLayer of type polygon or multipart polygon. Areas outside of this boundary will be assigned nodata.
output_path - the place in the filesystem where the dataset should be written. ⚠️ If it already exists it will be overwritten without confirmation.
crs - typically a UTM zone for the analysis area. Should be m based.
cellsize - based on the analysis environment, typically 100m
input_polygons - input polygons vector layer

Outputs:

a QgsRasterLayer
The analysis will generate a new raster layer in the input CRS and dimensions of the input bbox and cell size. The non land pixels will be assigned nodata. Land pixels will be assigned the value of the index_value.

Testing:

Prepare a small sample dataset for the boundary and verify that the checksum of the generated raster matches.