Wire up specset conversion for to_silo

[JustinPrivitera]

once #1349 is complete

See this note:

conduit/src/libs/blueprint/conduit_blueprint_mesh_matset_xforms.cpp

Line 1585 in 242abfb

// TODO change this once we write specset converters

[JustinPrivitera]

we will want this to be as fast as possible, so we will want to write three converters, one from each specset representation to silo. Ignore the comment in the code.