Rename / rethink `get_onboard_QC_values` and `get_onboard_QC_df`

[MichaelClerx]

• Second method gives same data as first, but drags in pandas as a dependency. If users want it in this form, can easily do it themselves based on 1st method?
• Names probably shouldn't include "QC" just because pcpostprocess uses thes values for that. Could maybe be more informative name? get_cell_parameters ?
• If df method stays you'd expect a _values in there for consistency?

[MichaelClerx]

pcpostprocess doesn't use the _df method, so removing