Add rescaling of reference scales in EchemdbEntry

[DunklesArchipel]

Checklist

• Added an entry in doc/news/.
• Added a test for this change.
• Adapted the documentation for this change.

Fixes #101

So far we have the following reference electrodes in the echemdb database for aqueous systems, which have to be accounted for:
['RHE', 'SHE', 'SCE', 'Ag/AgCl', 'Hg/HgO/0.1M NaOH']

• Ag/AgCl might pose a problem, since the concentration is not given.
  • Add a custom AgAgCl electrode with a shift of 210 mV and raise a warning?
• Hg/HgO/0.1M NaOH must be implemented. As long as we do not deconvolute the names, we should for the few cases simply add the offset.
• Check all references (source) for the shifts in the potentials and provide a DOI.
• move offset to unitpackage.entry.Entry