make FFTW a soft dependence

[johnnychen94]

This can reduce loading time ~50%:

• This PR: 1.280366 seconds (1.50 M allocations: 87.844 MiB, 0.82% gc time)
• Before: 2.707802 seconds (6.70 M allocations: 350.225 MiB, 1.67% gc time)

I do have two questions wrt Requires:

• will indirect loading of FFTW( e.g. using ImageFiltering) trigger this?
• does loading order matters? (e.g., using ImageCore; using FFTW vs using FFTW; using ImageCore)

My understanding is yes to both.

[johnnychen94]

In the nightly test there's a package warning:

┌ Warning: Package ImageCore does not have FFTW in its dependencies:
│ - If you have ImageCore checked out for development and have
│   added FFTW as a dependency but haven't updated your primary
│   environment's manifest file, try `Pkg.resolve()`.
│ - Otherwise you may need to report an issue with ImageCore
└ Loading FFTW into ImageCore from project dependency, future warnings for ImageCore are suppressed.

[RalphAS]

You can suppress the warning by using

import .FFTW

in src/functions.jl (note the "dot")

[codecov]

Codecov Report

Merging #106 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
+ Coverage   66.19%   66.27%   +0.07%     
==========================================
  Files           9        9              
  Lines         426      427       +1     
==========================================
+ Hits          282      283       +1     
  Misses        144      144

Impacted Files	Coverage Δ
src/functions.jl	100% <ø> (ø)	⬆️
src/ImageCore.jl	63.15% <100%> (+2.04%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a66027f...1795320. Read the comment docs.

[timholy]

Great idea!

I do have two questions wrt Requires:

• will indirect loading of FFTW( e.g. using ImageFiltering) trigger this?

Yes

• does loading order matters? (e.g., using ImageCore; using FFTW vs using FFTW; using ImageCore)

No