Added focal standard deviation filter

[mwip]

I missed a standard deviation filter. The raster::focal version is not usable at all for large data sets.

[codecov-io]

Codecov Report

Merging #38 into master will decrease coverage by 4.9%.
The diff coverage is 1.57%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
- Coverage   50.02%   45.12%   -4.91%     
==========================================
  Files          21       23       +2     
  Lines        1765     1724      -41     
==========================================
- Hits          883      778     -105     
- Misses        882      946      +64

Impacted Files	Coverage Δ
R/velox_focal.R	0% <0%> (ø)	⬆️
src/stddev.cpp	0% <0%> (ø)
src/integral.cpp	1.12% <1.12%> (ø)
src/focal.cpp	1.12% <4.76%> (-0.33%)	⬇️
R/velox_rasterize.R	73.91% <0%> (-3.14%)	⬇️
R/velox_cast.R	55.26% <0%> (-2.88%)	⬇️
R/velox_extract.R	51.7% <0%> (-1.84%)	⬇️
R/velox.R	85.1% <0%> (-1.43%)	⬇️
R/boostgeom.R	53.57% <0%> (-0.82%)	⬇️
R/velox_aggregate.R	0% <0%> (ø)	⬆️
... and 5 more

---

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[mwip]

What does this even mean? I never changed the respective source files. Please let me know, if this poses any issues to the merge and how to solve it.