numexpr is required
Rewrite of iec 61672

- fast and slow now only do the integration. fast_level and slow_level
integrate and return a level
- fix integrate. Conversion from zpk to tf was missing
- improve integrate and average. Both work with at least 2D arrays now.
Fix rms
Window scaling factor axis
Support higher dimensions when calculating spectra.

With higher dimensions is meant performing the same operation for
multiple rows or columns and not an N-D transformation.
Improve docstrings
Tests for iec 61672
Tests for ms and rms
Rewrote Signal class

- Can now contain multiple channels
- Improved plot titles