Printing hall of fame should be nicely formatted

[MilesCranmer]

This seems like a pretty obvious tweak to make: the string representation for a dominating Pareto front: https://github.com/MilesCranmer/SymbolicRegression.jl/blob/master/src/HallOfFame.jl#L112

should be used to define a custom show method for the HallOfFame type, so that users can print it without manually coding it up.

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Also, it might be nice to store HallOfFame in a DataFrames.jl type.