Fork operator [was separate]

[pditommaso]

Since the deprecation of create channel factory, it's needed an operator to replace the existing separate operator which requires the target channel to be created ahead.

For example:

c.fork { 
  foo: 
    <expression statement> 
   bar: 
     <expression statement> 
   .. : 
     <expression statement> 
} 

The closure returns an n-tuple of values each of which corresponds to the last evaluate expression statement in the corresponding block-labelled block.

Related Related #1281 #1269

[pditommaso]

Done, I've added some docs https://github.com/nextflow-io/nextflow/blob/6f89833a6ad4b5cd68633b7dcbf0a39daa3f5cdb/docs/operator.rst#fork

cc @micans

[pditommaso]

This has been renamed to multiMap. See afd671c.