Replace input/output set type with tuple type

[pditommaso]

The set input/output qualifier allows the definition of tuples of input and outputs. For example:

process foo {
  input:
  set val(x), file(sample) from x
  output:
  set val(x), file('*.bam') into z

  """
  the_command
  """ 
}

The set is misleading because it tends to be read as a verb, while the original intention was to intend it as name i.e. a set of value.

The goal of this issue is to replace it with the tuple keyword which provides a better definition of the associated data structure.

process foo {
  input:
  tuple val(x), file(sample) from x
  output:
  tuple val(x), file('*.bam') into z

  """
  the_command
  """ 
}

The semantic should remain the same as the current set definition.

[pditommaso]

Reminder to myself: add the docs.