Grid boundary setting

[mijinyoon]

The main reason to use grid setting is to test the case without any blending effect. But, the current implementation generate objects on the same image with possible contamination from neighbor objects. To avoid the contamination, each object generation should be conducted within each boundary after copying from the pixels from the whole image. Use boundary option from galsim function. Check demo10 from galsim (https://galsim-developers.github.io/GalSim/_build/html/tutorials.html)

[lshuns]

That is a good point. I have adjusted the pipeline to fix the stamp size for galaxies in grid mode, which avoids any blending (see Commits on Jun 16, 2021). A 'grid_size' parameter is added for stamp size adjustment.