isInitial function

[isaacsas]

It would be nice to have an official API function to detect if a symbolic is an Initial(X) for some symbolic X. We need this in places to filter out such variables.

I know we can use something like the following to detect and remove them from a vector

function remove_inits(ps)
    filter(x -> !iscall(x) || !isa(operation(x), Initial), ps)
end

but it would be nice to have an official function in case the internal representation for them changes at some point.

[ChrisRackauckas]

Agreed this seems pretty necessary. @vyudu can you prioritize this?