Add as method to align quantities of the same dimension

[zainab-ali]

Fixes #24

• adds an as method to enable users to align quantities of the same dimension
• adds some more doctests

[codecov-io]

Codecov Report

Merging #59 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
+ Coverage   85.99%   86.04%   +0.05%     
==========================================
  Files          10       10              
  Lines         257      258       +1     
==========================================
+ Hits          221      222       +1     
  Misses         36       36

Impacted Files	Coverage Δ
src/main/scala/libra/Quantity.scala	79.71% <100%> (ø)

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