Simplified the waveform generator:
* renamed 'parameter_keys' -> 'parameters'
* WaveformGenerator instances no longer have all the simulation parameters attached. They are now all contained in a single dict called 'parameters'
* parameters does not require complicated setting rules
* frequency_domain_strain() no longer requires lambda expression, parameters are now stored in the 'parameters' container
* Changed the set_values(dictionary) syntax a bit