Add feature to extern fitsio crate to support rsfitsio

[cruzzil]

fitsio crate https://crates.io/crates/fitsio

Add a feature to the fitsio crate to link against / build with rsfitsio

[cruzzil]

@simonrw let me know if this idea would be suitable for your project

[simonrw]

@simonrw let me know if this idea would be suitable for your project

I think it is a good target to aim for a 100% Rust fits implementation with a high level API 👍

[cruzzil]

Ive got a working prototype of this, but probably need a few more weeks before it's ready. One of the problems I need to think of a way around is the differing C-API that I use.

E.g instead of '*mut *mut fitsfile' I use '*mut Option<Box>' to better explain the intent whilst being FFI compatible.