Added level units.

- Level is the logarithm of the ratio of a quantity Q to a reference value of that quantity, Q0.
- Level is expressed in two dimensionless units: decibels (dB) and nepers (Np).
- Level is a non-standard SI unit but its usage is accepted alongside SI.
Merge pull request #47 from bretthysuik/feature/level

Feature: Level

    Level is the logarithm of the ratio of a quantity Q to a reference value of that quantity, Q0.
    Level is expressed in two dimensionless units: decibels (dB) and nepers (Np).
    Level is a non-standard SI unit but its usage is accepted alongside SI.