write proper testthat tests, add coverage indicators

2019-05-28:

> show(covr::package_coverage())
tidyfun Coverage: 63.04%
R/approx.R: 0.00%
R/geom-capellini.R: 0.00%
R/geom-spaghetti.R: 0.00%
R/gglasagna.R: 0.00%
R/graphics.R: 0.00%
R/print-format.R: 0.00%
R/smooth.R: 0.00%
R/concatenate.R: 35.51%
R/brackets.R: 40.66%
R/summarize.R: 50.00%
R/tfb-class.R: 50.00%
R/ops.R: 57.00%
R/math.R: 58.82%
R/methods.R: 65.15%
R/utils.R: 67.54%
R/tfb-spline.R: 81.12%
R/convert.R: 85.71%
R/tfd-class.R: 86.13%
R/evaluate.R: 87.50%
R/calculus.R: 88.62%
R/tidyr.R: 89.33%
R/depth.R: 94.44%
R/zoom.R: 95.00%
R/tfb-fpc.R: 95.89%
R/tfb-utils.R: 97.19%
R/where.R: 97.50%
R/rng.R: 100.00%

tidyfun / tf

write proper testthat tests, add coverage indicators #19