Add ability to align interfaces for multi-model

[welbournR]

The align interfaces option in the profile uncertainty tab doesn't work to align the curves when there are multiple models in the experiment. This can happen frequently for soft matter experiments (and can include a mix of back_reflectivity on/off).
It looks like the interfaces get assigned correctly with the vertical dashed lines but there is no shift in the z-axis to align these. For example:

Suggestion to handle the zero-point to be flexible across a range of scenarios: the zero point becomes the position of the dashed lines so "z" now corresponds to the depth relative to the chosen aligned interface (rather than the bulk interface).