Guides: explainer for writing channels

[georgiesamaha]

Writing channels is not adequately explained in Nextflow docs. Writing channels often requires complex groovy syntax when joining different outputs. Our guide should include an approachable set of instructions for common channel types. Some examples of things we should cover:

• Tuples connecting sample name to specific files
• Connecting reference files with their indexes
• Joining multiple files output by separate processes at the sample level
• Joining different sample files output by individual tasks for the same process

[georgiesamaha]

To include:

• Extracting information from samplesheets
• Grouping multiple channels by tuple (e.g. sample id)
• Using view() to build and test a channel
• Operators + groovy