Add replicate support for metatranscriptomics

[AmandaBirmingham]

As of 2025-08, the wet lab has never done any metatranscriptomics with replicates, and it turns out the code for this doesn't actually fully exist (or rather, replicate support code was added to the metat notebook but not fully to the metat sample sheet). To support metat replicates, it will be necessary to

• create and test (in both replicate and non-replicate cases) a new v20 metat sample sheet
  • might as well add SampleContext support while at it
• comment back in replicate support in the metat notebook and update the sample sheet version it creates