Osc-gen

This release includes:
* Many documentation updates in the README file.
* Addition of a 'mix' parameter in the 'sqr_saw' method in SigGen.
* Improvements to plotting layout.
* Some improvements to error handling in WaveTable.
* Addition of 'to_wav' and 'to_h2p' methods in WaveTable to facilitate easier conversion of wavetables to output files.

Add new waveform methods in sig module for skarkfin, filtered noise and exponential saw and sin waves.

* Add a morph_with() method to allow two wavetables to be morphed together.
* Add init parameters for SigGen() and WaveTable() to more easily set the number of cycles/waves.
* Fix end-of-cycle and default phase behaviour to ensure zero-crossings are lined up correctly.
* Improve plotting functions in the visualize module.

* Add a morph_with() method to allow two wavetables to be morphed together.
* Add init parameters for SigGen() and WaveTable() to more easily set the number of cycles/waves.
* Fix end-of-cycle and default phase behaviour to ensure zero-crossings are lined up correctly.
* Improve plotting functions in the visualize module.

Minor clean up of style and more examples.