DDH5Writer needs a way to add parameter metadata after the experiment is done

[hatlabcz]

Some experiments doesn't have a known list of parameter values at the beginning of the experiment (e.g. list of timestamps when each run is done). These values can be collected during the experiment, so we do know them at the end of the experiment. We need a method to add these values as metadata after the experiment is done.

[hatlabcz]

This is solved in the last commit of Hatlab_DataProcessing (PITT-HATLAB/Hatlab_DataProcessing@61b219a).
To add a value to the metadata of a outer_sweep parameter "param", we can do: Experiment.ddw.add_meta(val_param=value).

Here, the prefix "val_" is needed to label this metadata as values of a sweep parameter (check source code of the Experiment class for details)