Handling of real-valued / causal / acausal / anticausal filters

[chipmuenk]

Accuracy could be improved by specifying the basic properties of a filter via checkboxes:

• Real-valued filter: only e.g. P / Z with positive imaginary part are calculated / entered in the P / Z editor, the negative ones are constructed automatically. This prevents coefficients with small imaginary parts during conversion P/Z -> coefficients
• Causal filters: The correct number of P / Z at the origin is added / deleted automatically
• Anti-Causal filters: use separate algorithms for calculating group delay, impulse response etc.