Full MTNG support

[cbyrohl]

Preliminary MTNG support

• unit discovery
• doc page
• load snapshot
  • regular "snapshot_XXX" snapshot
  • "snapshot-prevmostboundonly_XXX" snapshot
  • "snapshot-prevmostboundonly-treeorder_XXX" snapshot
• load catalog
  • regular "fof_subhalo_tab_XXX" catalog
  • "subhalo_prog_XXX" catalog
  • "subhalo_desc_XXX" catalog
  • auxiliary "subhalo_treelink_XXX" tree catalog
• load merger trees
  • regular "trees"
• load light cones