Support for units in nat

[dokato]

To avoid problems like: https://github.com/flyconnectome/malevnc/issues/16

Suggestion for a package that might help: https://cran.r-project.org/web/packages/units/index.html

Steps:

• [ ] extra field in neuron object
• [ ] neuronlist?
• [ ] extracting unit from metadata while loading neurons
• [ ] introduce unit check for operations like +, /, *, etc.

[dokato]

thanks for the nudge @jefferis , I started tinkering something with units, there's one serious problem though: https://github.com/r-quantities/units/issues/314