Unify `CTD` and `CTD_BGC` to one instrument

[j-atkins]

CTD and CTD_BGC are probably best unified - in reality they are the same instrument.

Open question about whether there should then be more configuration options where different variables can be switched on/off.

[ammedd]

Yes, I think unifying them and leaving the variable choice to the user makes sense. This would exemplify a scientist deciding which sensors need to be on the CTD frame for their research.

[erikvansebille]

Nice idea! In the future, there could maybe even be the option for an LADCP on a CTD?

[ammedd]

Definitely! Has been on the wishlist since #28