Only support tuples and not single inputs/outputs

[gdalle]

The ambiguity between tuples and single inputs/outputs in pushforwards and pullbacks is causing several bugs (#99) and making the code more complicated.

I propose to make a breaking change that supports only tuples

cc @devmotion

[devmotion]

An alternative would be https://github.com/JuliaDiff/AbstractDifferentiation.jl/issues/53

[gdalle]

Good point. How to decide between the two? Would one be

• easier to code?
• more likely to be type-stable?

[gdalle]

On second thought they're actually the same, replacing args... with args everywhere does the translation

[mohamed82008]

I think it should be fine to only support tuples. I don't recall why we support both but we can just support one.

[gdalle]

Only supporting plain inputs / outputs as in #53 sounds slightly more natural though, doesn't it?

[mohamed82008]

that's also fine by me

[gdalle]

Closing this in favor of #53 then