Adding rules for rem2pi, mod2pi

[tkoolen]

To get both rem2pi and mod2pi, it should be sufficient to just define a rule for rem2pi, as mod2pi is implemented in terms of rem2pi in Base. Unfortunately, rem2pi, with its RoundingMode argument, doesn't really fit the mold of functions accepting only numeric arguments (and thus having derivatives with respect to all of the arguments). I also don't really think it makes sense to define rules for the lower-level functions that are called in rem2pi (e.g., ieee754_rem_pio2, add22condh), as those seem more like implementation details.

Do you have any plans for how to handle this case? I can help implement this, but if you could give me an idea of what the required changes to DiffBase (and possibly dependent packages) would be, that would be very helpful.

Ref: JuliaDiff/ForwardDiff.jl#113.

[jrevels]

If a derivative for an argument doesn't exist/make sense, my general strategy has just been to pop a NaN in there (see the bessel* diffrules, for example).

ref JuliaMath/Calculus.jl#89

[tkoolen]

OK great, that's what I'll do then.