First beta for 0.2 release

• removed Python 2.x compatiblity (from __future__, six etc.)
• removed myhdl fixpoint filters and dependencies
• added Migen interface to pyfda and a DF FIR filter as a proof of concept. Migen is not a dependency yet, fixpoint functionality is only available if migen is available in the system path. If not, the fixpoint widget is deactivated silently
• new parser for config file with module names instead of class names for all widgets. Fixpoint filters can be associated with filter designs and vice versa
• removed deprecation warnings and fixed compatibility with Matplotlib 3.1
• fixed floating widget in plot_impz.py