Add ringdown waveforms

Minke currently can't produce ringdown waveforms; these were previously a component of standard MDC sets, though we have dropped them in the advanced era.

• Write unit-tests for generation of non-SimBurstTables
• Add support for tables which are not SimBurstTables to mdctools
• Add support for generating ringdown waveforms
• Add API-level documentation
• Add human-readable documentation
• Add information to features etc on README