new variable auto restart

[nickhsmith]

The issue is that params is a nextflow variable that can't be overridden in the script. When we wrote params.input = csv_from_old_run It doesn't actually update the params map.

However if we introduce a new variable params.step_input = csv_from_old_run if params.input is null or "" AND the mapping step matches. Then we can have a check

input_csv = params.input ? params.input : params.step_input
ch_input_sample = extract_csv(file(input_csv, checkIfExists: true))    

csv_input will be the same as params.input if it is non null/empty. OR it will be the csv determined by the retrieveInput function