allow symbolic indexing of vectors of observables

[vyudu]

fix #38

[MasonProtter]

We'll also want a method in ext/MTKExt.jl that converts Nums to Symbols, i.e.

function SymbolicIndexingInterface.observed(sys::PartitionedGraphSystem, vars::Union{Vector{Num}, Tuple{Vararg{Num}}})
    SymbolicIndexingInterface.observed(sys, tosymbol.(vars; escape=false))
end

[vyudu]

@MasonProtter is this waiting on something?

[MasonProtter]

Oops, im sorry I forgot about this! Could you just change the test so that it gets run by CI?

It's currently just in a free-floating file i think

[vyudu]

fixedup, should be mergeable