implement Zero for Quantity when the backend supports Zero

[radix]

Here we go, my first try at a trivial trait implementation for uom.

Please let me know if there are any problems or improvements I can make!

[coveralls]

Coverage increased (+0.01%) to 98.552% when pulling ecaab25 on radix:zero into e05dab3 on iliekturtles:master.

[coveralls]

Coverage increased (+0.02%) to 98.559% when pulling 3fa3c46 on radix:zero into e05dab3 on iliekturtles:master.

[iliekturtles]

Implementation looks good. Some minor changes then I'll merge!

• Squash the commits. Change the message to something like "Implement num::Zero.\n\nPart of Add zero constants/methods for each quantity #26."
• Move the implementation in system.rs below RemAssign.
• tests.rs
  • Put a blank line beneath let z = ...
  • Use Test::assert_eq(V::Zero, z.value);

[radix]

FYI I believe you can squash the commits as part of merge with github while you're merging it, though it may be a setting that needs to be enabled in your repository. Regardless I will squash as you requested :)

[radix]

hooray, forgot to run git add before git commit --amend ;P this test pass should hopefully work...

[coveralls]

Coverage increased (+0.02%) to 98.559% when pulling b45948b on radix:zero into e05dab3 on iliekturtles:master.

[iliekturtles]

Replaced literal 0.0 with V::zero() and merged. Thanks!