[RFC] Get depvars and indvars in the right type for MOL

[xtalax]

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This change reflects something that is done internally in MethodOfLines.jl, which may have more general applicability.

Gets all dvs and ivs in their naked Symbolics.jl type, which is more useful for applying rules based on SymbolicUtils.jl.

What do you think of this @zoemcc @YingboMa?

I will add tests if this is decided to be a good change.

[xtalax]

My apologies, this was copied over from code that was written before I was working on this. I will edit the style to be more in line with the rest.