addition of tmt18plex as labelling type and Average Channels as Reference Option

### Description of feature

I would like to request the addition of a feature to support TMT18plex as a labeling type. Currently, the largest plex option is TMT16plex, so if more than 16 labels are provided as in the case of TMT18, the last two are dropped.

Additionally, it would be highly beneficial to provide an option to calculate ratios using the average of all channels as the reference channel, rather than requiring a single specified reference channel. This feature would enhance flexibility and allow for better normalization in cases where no single reference channel is appropriate. This is often present in the literature.