Rename isconvertible -> isconcretizable

[gaurav-arya]

More of a mouthful, but the consistency seems worth it

[gaurav-arya]

On the other hand, it's not a word :/ OK this is just bikeshedding for now

[gaurav-arya]

https://en.wikipedia.org/wiki/Concrete_category#Definition

[vpuri3]

what's the final word?

[gaurav-arya]

I think we should wait until the WOperator PR, for isconvertible to get some mileage in OrdinaryDiffEq. As I think there's still some more thinking to do on the semantics, which would of course inform the name