Support the strided array interface

[dlfivefifty]

Note that Julia supports a strided array interface. At the moment FFTW only works with StridedArray which means other types can't use it, see below example.

I assume this is an easy fix and just replace ::StridedArray with ::AbstractArray, as non-compatible types will error out on the call to pointer.

julia> using BlockArrays

julia> n = 10; x = PseudoBlockArray(randn(n), [5,5])
2-blocked 10-element PseudoBlockVector{Float64}:
  0.13895184669862432
  1.0296194237071945 
  0.831728129597864  
 -1.2760605104194878 
  1.225360630804565  
 ────────────────────
  2.9373002173159604 
 -0.20348560029959115
  1.3919462067161756 
 -0.5452870826131355 
  0.6774104064065777 

julia> strides(x)
(1,)

julia> pointer(x)
Ptr{Float64} @0x0000000141ab5770

julia> F = FFTW.plan_r2r(randn(n), FFTW.REDFT00);

julia> lmul!(randn(n), F, x)
ERROR: MethodError: no method matching lmul!(::Vector{Float64}, ::FFTW.r2rFFTWPlan{Float64, (3,), false, 1, UnitRange{Int64}}, ::PseudoBlockVector{Float64, Vector{Float64}, Tuple{BlockedUnitRange{Vector{Int64}}}})
Stacktrace:
 [1] top-level scope
   @ REPL[9]:1