mergeSEs: improve

[TuomasBorman]

Add parallelization to the method. Also, add option for choosing whether to combine colData columns even though their classes differ

Currently, the method checks if equally named columns have same class --> if not, do not merge. (There could be an option that allows method to merge columns even though they have different class)

[antagomir]

Also consider renaming mergeSEs -> combineSEs since now we are also using merge for row/col agglomeration.