Add Barn, improved error messages

[Vindaar]

Some minor changes, with one breaking change. Renaming G for Gauss to gauss, due to potential problems with Giga SI prefix.

The version bump to v0.4.0 is mainly as this is the state used for the results in my thesis.

Full changelog:

* v0.4.0
- add unit of ~Barn~
- BREAKING: rename short form of Gauss to ~gauss~. Single capital ~G~
  is too brittle combined with Giga prefix
- improve printed unit names in error messages
- add hbar as a constant
- add note about natural unit convention used