Add curios slot for tanks

[bynect]

Now oxygen backtanks can be equipped in the extra back slot of curios api.

ideally the backtank and the spacesuite should be different item that can be optionally combined, so you can have the tank in the slot and the chestplate as normal. or I guess the tank slot could be an extra oxygen supply

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[RealAntEngineer]

there is one issue : how does it looks when you move the backtank into the curios slot ?

[bynect]

at the moment the tank is rendered but the armor piece not

[bynect]

ideally the rendering of the tank and the armor piece should be disconnected totally