use digits=5 (not sigdigits) to generate nice random numbers.

[oxinabox]

Fixes #206

There were 3 problems:

• sigdigits was used rather than digits which meant that numbers like 0.0000000000001234 were showing up.
• the rng which was in the test to reduce chance of hitting edge cases unexpectedly wasn't used
• that rng wasn't initialized so didn't actually serve that purpose anyway

I have now run these tests a few million times, rather than the 50 that is there, and it always passes now.

[codecov-commenter]

Codecov Report

Merging #207 (6eb3529) into master (ae32599) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #207   +/-   ##
=======================================
  Coverage   91.00%   91.00%           
=======================================
  Files          11       11           
  Lines         289      289           
=======================================
  Hits          263      263           
  Misses         26       26           

Impacted Files	Coverage Δ
src/rand_tangent.jl	100.00% <100.00%> (ø)

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[simeonschaub]

Thanks! LGTM.