Update Demultiplexing process

[golharam]

• As a user when I want to demultiplex a run, I want to specify an assay for each sample.
  • For multiple types of assays - allow user to enter assay without validation in Sample_Assay column in the samplesheet
  • For only one type of assay - select an assay drop-down list, globally for all samples, but include a generic assay so as to not prevent the user from moving forward. (Current workflow).