Suggestion: add the choice of H&E stains to nuclei segment instead of DAPI stains

[falsetry1514]

Hi, BIDCell Team.
For some Sequencing-based ST method, the DAPI stain wasn't recommend to do. Instead of DAPI, they use H&E stain to do ST pipeline, such as the new tech, openst. So Could you add a patch for H&E stain or just provide the nuclear mask by user to jump the nuclei_segment step. I think it will much better for multiplex demand.