Fixes virtual file merge logic & removes deprecated distributed set merge logic#206

Draft

evanhanders wants to merge 3 commits intoDedalusProject:masterfrom evanhanders:merge_fixup

+49-185

Commits on Jun 16, 2022

removes metadata that no longer exists (at least in virtual files)
evanhanders
committed

Commits on Jun 20, 2022

test_output_parallel.py now properly tests mpio, virtual, gather, and merged-virtual
evanhanders
committed
removes deprecated distributed_set merging, makes virtual file merging easier to read
evanhanders
committed