Description of units

[danielwilms]

As discussed in #151 the description of units is very short. Therefore we should think:

1. If a url pointing to an official unit spezification could be an alternative to the description
2. If a url pointing to an official unit spezification could be an addition to the description
3. If we want to have more detailed descriptions ourselves

[erikbosch]

Do we generally see a need to have more detailed unit descriptions? I can see that there in some cases are ambiguities, but for a general example like "Mass measured in grams"; would we actually benefit from referencing e.g. the wikipedia article on Gram? Isn't it sufficient to resolve ambiguities, e.g. to replace "Mass measured in pounds" with "Mass measured in international avoirdupois pounds"