Some enhacements in the documentation

[Enchufa2]

• Add some notes about temperatures in Ops.units; Arithmetic ops with offset units (e.g. temperature) should issue a warning #253
• Refactor documentation abount unit creation and conversion; Units conversion is burried in the docs #200
• Add some notes about numerical prefixes; as_units("10 m") vs. as_units("10m") #90

[codecov]

Codecov Report

Merging #257 (13bc4ac) into master (cc1911f) will not change coverage.
The diff coverage is 85.71%.

@@           Coverage Diff           @@
##           master     #257   +/-   ##
=======================================
  Coverage   94.20%   94.20%           
=======================================
  Files          19       18    -1     
  Lines         914      914           
=======================================
  Hits          861      861           
  Misses         53       53           

Impacted Files	Coverage Δ
R/arith.R	96.87% <ø> (ø)
R/make_units.R	90.59% <81.57%> (-4.81%)	⬇️
R/conversion.R	88.23% <94.44%> (+3.47%)	⬆️

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[edzer]

Looks good to me, thanks!!