Support for logarithms

[Enchufa2]

Initial implementation to address issues in #176.

[codecov]

Codecov Report

Merging #177 into master will increase coverage by 1.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #177      +/-   ##
==========================================
+ Coverage   98.44%   99.48%   +1.03%     
==========================================
  Files          18       18              
  Lines         964      968       +4     
==========================================
+ Hits          949      963      +14     
+ Misses         15        5      -10

Impacted Files	Coverage Δ
R/math.R	100% <100%> (ø)	⬆️
R/options.R	100% <100%> (+4.83%)	⬆️
R/init.R	100% <100%> (ø)	⬆️
R/valid_udunits.R	100% <0%> (+5.64%)	⬆️

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[Enchufa2]

@edzer, @t-kalinowski: I think this PR is ready. Please, take a look. Summary:

• Logarithms now work via udunits2.
• BW (bel-Watts) and others are defined by udunits2, but not B (bel). With this PR, by default B is added, unless a user-provided XML database defines some B already or it's disabled via the define_bel option.
• Some tests and docs (see help(Math.units)) about the behaviour of the logarithm have been added.

If you agree with the changes so far, we can merge this PR and close #176. Maybe the best is to avoid any automatic conversion (to dBW, etc.) at the moment: we can always revisit the discussion in #176 if the users demand that feature.

[t-kalinowski]

Looks good to me!

[t-kalinowski]

By the way, I find your recent comments in #176 convincing, and I think it's probably OK to do automatic conversion.

[Enchufa2]

Nooo... Did we convince each other? :D :D :D

[Enchufa2]

Ok, I've added a couple of tests for the decibel too. May I squash and merge this, @edzer?

[edzer]

Yes, very good - go ahead. Great work!!