Add quaternion SpecialFunctions?

[sethaxen]

With #56, we would be able to extend a number of special functions to the quaternions. Currently, via DualNumbers, this package already depends on SpecialFunctions, so it wouldn't increase the weight of this package at all. The question is, would this actually be useful?

[sethaxen]

Using the heuristic in #56 (comment), these are the unary special functions that maybe could be extended:

julia> get_complex_analytic_funs(SpecialFunctions)
11-element Vector{Symbol}:
 :airyai
 :airyaiprime
 :airyaiprimex
 :airyaix
 :airybi
 :airybiprime
 :besselj0
 :besselj1
 :bessely0
 :bessely1
 :expintx

There are of course also binary functions like bessel that could be extended.