eclab: Rework uncertainty handling

[PeterKraus]

• The uncertainty handling in the eclab extractors now works in two stages:
  • first, the uncertainty in voltage and current is determined using the previous method, by considering the I and E ranges.
  • second, the relative error in V and I is calculated
  • third, the uncertainty of derived quantities is calculated by considering the formula required to obtain the derived quantity
• Added a fallback for mpt files, where uncertainty might evaluate to that determined from reading the string representation of the files.

Will close #156.