Implement better tile aggregation

[AjeyPaiK]

This PR implements StitchingMode.AVERAGE along with StitchingMode.MAXIMUM in ahcore.readers for better tile stitching.

Functionality of the stitching modes illustrated through a test case:

1. We make a 2x2 grid with tiles of size (200,200) with an overlap of (50,50).
2. Each tile has a different color and at the overlaps, the colors are mixed as shown below.
3. In the overlap regions, the portion where two tiles overlap has a value of 4, the portions where two tiles overlap has a value of 2. The black regions have a value of 1.
4. The overlap regions array is used for the averaging operation or to calculate maximum values.
5. The test I have implemented checks for colors in the different overlap regions and non-overlapping regions of the stitched image and asserts for correctness.

Stitched Tile	Overlap Regions

What is the effect of this on the quality of segmentation outputs? Check below.

Before	After Averaging	After Averaging with large tile_overlap