Support LB iterations

[ppebay]

This is related to vt#2375

@lifflander if you want to add something

In the JSON for a phase:

"phases":
  [
     {
       "id": ...,
       "tasks": ...,
       "communications": ...,
       "lb_iterations": [
         {
           "id": ...,
           "tasks": ...,
           "communications": ...
         }
       ]
     }
  ]

[ppebay]

@lifflander should the writing of intermediate load-balancing iterations occur in offline_LB_compatible mode only, or in all cases? If I am not mistaken, the former is what is typically used when LBAF processes pure vt outputs.