Add multidimensional interface for lib transforms

[dlfivefifty]

th_cheb2leg and th_leg2cheb support FFT like interface, eg., for a matrix fft(X, 1) transforms the 1st dimension, fft(X, 2) the 2nd and fft(X) == fft(X, (1,2)) == fft(X, 1:2) transforms both.

What to do about parameters though? Should it be jac2jac(X, a, b, α, β, 1) to transform the 1st dimension or jac2jac(X, 1, a, b, α, β)?

Or should we be using tuples here: jac2jac(X, (a, b), (α, β), 1)?

[MikaelSlevinsky]

Probably the OP parameters should trail region, since they're like FFTW R2R flags, which are at the end.

[MikaelSlevinsky]

Oh I was looking at the C API of FFTW, not the Julia API.

[dlfivefifty]

Ok then that supports putting the region at the end.

Unfortunately it probably means we'll have to implement plan_*(::AbstractArray, α,β ,…,ω, region) for each transform spelling out the parameters as I don't see how in the general to tell if the last variable is meant to be a (possibly integer) parameter or a region