Only print non-default params in class repr

[rxavier]

• Add the poniard.utils.utils.get_kwargs function, which is used within class __init__ to parse construction parameters.
• Add the poniard.utils.utils.non_default_repr function, which takes a class instance, checks its init_params attr (obtained from the above), compares with the __init__ default params, and produces a repr which only contains non-default parameters.
• Modified PoniardBaseEstimator, PoniardPlotFactory and BasePlugin to use these reprs.
• Custom plugin devs should simply include super().__init__() within initialization to get the new repr.