Add demuxmix functions

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`](https://bioconductor.org/packages/release/bioc/html/demuxmix.html), which uses a mixture model to better model the hashtag oligos. This method performed well in a recent review paper: [Howett et al. 2023](https://doi.org/10.1093/nargab/lqad086).

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

Review the existing demultiplexing functions in scpcaTools and the demuxmix package documentation first. Add the requested demuxmix functions and verify that they support the intended hashtag-oligo mixture-model workflow; evaluation for scpca-nf is explicitly a later step.

Written by the indexing model from the issue text.