Red channel workflow enhancements

[marysethomas]

In response to #747

This is my first contribution - please let me know if there is anything I could improve!

[marysethomas]

Update 12/10/21

This pull request now contains three modifications:

1. Save F_chan2 and Fneu_chan2 during save to mat
2. Add chan2, corr and chan2 as view options in Manual Labeling
3. Add the 'C' key as a shortcut to label/unlabel red cells

In response to issues #747, #779, and #528

[marysethomas]

Red cell shortcut is the same as pull request #786, I did not know how to select my other commits for this pull request without also selecting that one, my apologies!