Transformations don't transform the metadata["zerovalue"]

[ladc]

It would be useful if all transformations are fully reversable, but currently boxcox_transform, dB_transform and NQ_transform aren't.

In the boxcox_transform and dB_transform this just because the metadata["zerovalue"] does not use the same transformation as the data and the metadata["threshold"]. This was done because np.log(0) results in -np.inf, this should be easily mitigated by introducing a minimal value in the outcome, i.e. metadata["zerovalue"] = np.maximum(np.log(metadata["zerovalue"]), -1000). The same should maybe also be applied to the actual data that is transformed and the metadata["threshold"].

In the NQ_transform, both metadata["threshold"] and metadata["zerovalue"] are not reversable, so maybe we can apply the interpolation method to both of these parameters as well, not sure if this is possible though.