Enhanced output val declaration adding support for value literals and value expressions

[pditommaso]

Currently an output val declaration is able only to capture a value defined in the process script context.

However it'd be useful to allow to use also value literals and value expression. For example:

x = 100 

process foo {
  input: 
  file fastq from 'dummy'

  output: 
  val 'Hello' into str_channel 
  val "${fastq.baseName}-${x}.out" into exp_channel

  """
  echo bar 
  """
} 

str_channel.println { "str: $it" }
exp_channel.println { "exp: $it" }

It should print:

str: Hello
exp: input-100.out

[pditommaso]

This is related to issue #42

[pditommaso]

Released with version 0.15.0