add differential expression subworkflow

[an-altosian]

1. move the RANK_GENES_GROUPS module into the DIFFERENTIAL_EXPRESSION subworkflow.
2. do DE for both leiden clustering results and cell types detected.
3. modified LEIDEN, SINGLER and CELLTYPIST modules to make them write the cell labels into a cvs file, and do DE based on these files.
4. tested using the test profile.

TODO:

1. I am not familiar with multiQC so I skipped that part.
2. nf-core test needs to be updated.

[nictru]

Hey, just to let you know, I am currently on vacation and will look into this after that

[an-altosian]

Haha no worries at all! I was just too excited to stop coding yesterday. Maybe we can have a kick-off meeting when you come back from your vacation, because I modified a quite big amount of chunks of code.

Have fun!

Best,
Dongze

[an-altosian]

Hi @nictru , I think this PR is ready. Please let me know if you want me to explain this further!