Improve MPI Interface/Mechanics

[alfpark]

• [x] Add specialized sections to multi_instance for a subset of popular MPI stacks
  • [x] Intel MPI
  • [x] IB support
    • [x] Automatically expose proper files/devices for NetworkDirect RDMA?
    • [x] SR-IOV
  • [x] TCP support
  • [x] OpenMPI
  • [x] IB support
  • [x] TCP support
  • [x] MPICH
  • [x] IB support
  • [x] TCP support
• [x] Synergy with the existing num_instances option
• [x] Break out certain key options
  • [x] Processes per node
  • [x] Auto host list setup
• [x] Multi-instance task runner script
• [x] Lower friction for Singularity tasks
• [x] Update docs
  • [x] Base README
  • [x] Multi-instance doc

Additional MPI stacks (stretch goal):

• [x] MVAPICH
  • [x] IB support
  • [x] TCP support
• [ ] IBM Platform MPI
  • [ ] IB support
  • [ ] TCP support
• [ ] MSMPI (Windows only)
  • [ ] TCP support

This feature is related to #277.

[alfpark]

cc @vincentlabonte