Add demuxmix functions

[jashapiro]

Since we last looked at demultiplexing methods, a number of new methods and packages have come out that we might want to look into. As a first step, we can add functions to perform different multiplexing methods to scpcaTools.

The first of these we might want to look at is demuxmix, which uses a mixture model to better model the hashtag oligos. This method performed well in a recent review paper: Howett et al. 2023.

Following this addition, we can do some evaluations to see if we will want to add this method to scpca-nf.