TableTransformations should be a stack

[maxnoe]

Please describe the use case that requires this feature.
Currently we only support one column transform per column. However, this e.g. excludes adding units and using FixedPointTransform
which would be e.g. useful for peak_time.

Describe the solution you'd like

Transformations should be a stack per column.

[maxnoe]

Note: that's kind of reinventing (or reusing) a lot of fits conventions for the HDF tables