Create TruncatedNormalHyperparameter

[mfeurer]

To decouple the code for bounded and unbounded normal hyperparameters.

[eddiebergman]

@Neonkraft

The initial reaction was to have a heirarchy of NormalFloatHyperparameter -> TruncatedNormalHyperparameter but then checking for a non-truncated Normal becomes tedious.

x = NormalFloatHyperparameter(...)
y = TruncatedNormalFloatHyperparameter(...)

items = [x, y, ...]  # Some other hyperparameter types
for item in items:
    if isinstance(item, NormalFloatHyperparameter):
        ... # This would match both x and y

The correct solution would then be:

if isinstance(item, NormalFloatHyperparameter) and not isinstance(item, TruncatedNormalFloatHyperparameter):
    ... # This is the only way to match just x

I'm okay with some minor code duplication to keep these seperated, otherwise we need some _private superclass, which just makes the heirarchy of hyperparameters more complex.

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This would also constitute a pretty major breaking change and a version bump. This should probably be done at the same time as doing so for Uniform and possible Beta params

[eddiebergman]

We'll no longer have unbounded hyperparameters in #346 so that all hyperparameters are have explicit bounds.