More duration and memory unit syntax sugar

[pditommaso]

The goal of a this issue is to extend support for duration and memory unit syntax so that:

1. it is possible to define expression mixing numbers and memory or duration units as it's possible in the workflow script. For example:

   process.memory = { 1_000 * 1.GB }
   

2. Easily convert a number value to a duration or memory unit respectively using toDuration or toMemory methods. For example:

   1_000.toMemory() 
   3_600.toDuration()
   

3. Implicitly import Duration and MemoryUnit classes so that it is possibile to use them without the need to import explicitly or use the full package name.