Add freqkernel and spacekernel

[timholy]

This replaces #81. Comments welcome!

[codecov[bot]]

Codecov Report

Merging #100 into master will decrease coverage by 0.34%. The diff coverage is 65.38%.

@@            Coverage Diff             @@
##           master     #100      +/-   ##
==========================================
- Coverage   81.43%   81.08%   -0.35%     
==========================================
  Files           9        9              
  Lines        1174     1200      +26     
==========================================
+ Hits          956      973      +17     
- Misses        218      227       +9

Impacted Files	Coverage Δ
src/ImageFiltering.jl	80% <ø> (ø)	:arrow_up:
src/utils.jl	62.5% <65.38%> (+2.5%)	:arrow_up:

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[codecov[bot]]

Codecov Report

Merging #100 into master will increase coverage by 0.15%. The diff coverage is 88.46%.

@@            Coverage Diff             @@
##           master     #100      +/-   ##
==========================================
+ Coverage   81.43%   81.58%   +0.15%     
==========================================
  Files           9        9              
  Lines        1174     1200      +26     
==========================================
+ Hits          956      979      +23     
- Misses        218      221       +3

Impacted Files	Coverage Δ
src/ImageFiltering.jl	80% <ø> (ø)	:arrow_up:
src/utils.jl	73.21% <88.46%> (+13.21%)	:arrow_up:

---

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 61b04b4...c782b6c. Read the comment docs.

[timholy]

I'll merge this soon, barring any further feedback.