Rational dimensions

[Tehforsch]

Solves #25

This adds the rational-dimensions feature to use rational dimensions instead of just integer dimensions. Comes along with some minor code cleanups. I added my own very unsophisticated Ratio type in order not to pull in an additional dependency.

Todo:

• add documentation for the feature

• write better tests (ideally with a useful example)

• think about how to improve compiler error messages. Probably rewriting the Ratio struct to a tuple struct with two entries will already improve things.

• add tests to CI

• Allow definitions to make use of the rational dimensions so we can write dimension SqrtLength = Length^(1/2)