make Parameters object more flexible.

Moved Parameters object into utils and switched around where a few methods were delcared so that the same object can be used in EventGenerators, SamplingDistributions, and AttributeTransformations. This means that we should be able to pass either an EventGenerator or a SamplingDistribution to gwpopulations without issue.

Note, we have written EventGenerator, SamplingDistribution, and AttributeTransformation to raise KeyErrors when users try to access (either get or set) parameters that they don't know about. This is different than a dictionary, which will create a new key if one does not exist (when setting a key:value pair).