transform_unconstrained_to_constrained behavior is unclear for single values and Vectors

[nefrathenrici]

A single value results in an uninformative StackOverflowError, and a Vector results in just one value being returned.

transform_constrained_to_unconstrained has the same issue.

Reproducer:

import EnsembleKalmanProcesses.ParameterDistributions: ParameterDistribution, bounded_below, transform_unconstrained_to_constrained, transform_constrained_to_unconstrained

posterior = ParameterDistribution([1.;;], bounded_below(0), "test")

transform_constrained_to_unconstrained(posterior, 1.0)
transform_constrained_to_unconstrained(posterior, [1.0, 2.0])

transform_unconstrained_to_constrained(posterior, 1.0)
transform_unconstrained_to_constrained(posterior, [1.0, 2.0])