Merscope preprocessing fix

[jaspreetishar]

Update:

1. resolved the “cell_tile_{i}_{j}.parquet is missing" error by doing the following:

Instead of requiring fine tiles to be fully contained inside the coarse tile, i just checked for any overlap. also, filter_and_save_fine_boundary only writes a parquet if the fine tile has cells. now it writes a file even if it is empty

2. resolved the visual disappearance bug too. when loading the cbg from merscope, the indices were set as the cell-names ang they were of type int. i added a check to make sure they are of type string, so that when the cbg directory and subsequent gene parquet files are generated, the cell-names-to-int mapping gets done correctly.