Support for units in nat

[dokato]

To avoid problems like: natverse/malevnc#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: r-quantities/units#314