Add proper inversion of Measures

[atmoos]

There are a couple of Units and Quantities that are each other's inverse. Examples include:

• Frequency $\leftrightarrow$ Time
  • $\mathsf{Hz} \leftrightarrow \mathsf{s}^{-1}$
• Conductance $\leftrightarrow$ Resistance
  • $\mathsf{S} \leftrightarrow \Omega^{-1}$

In order to represent these relations consistently, the notion of inversion of units and measures is required.