`AbstractState` struct

[longemen3000]

when using PropsSI, a new abstract state is created with each call. one option would be to do something like:

mutable struct AbstractState #mutable, to register finalizers
  handle::Clong
  function AbstractState(x::Integer)
    res = new(Clong(x))
    finalizer(AbstractState_free,res) #register finalizer, to free up memory when `AbstractState` goes out of scope
    return res
  end
end

function AbstractState_free(x::AbstractState)
  return AbstractState_free(x.handle)
end

function PropsSI(val1,prop1,val2,prop2,fluid::AbstractState)
  # do low level call, fluid is mutated
end