single objectivePriorParameters are cast to numpy.float64

When all objectivePriorParameters are either empty or set to a single value, the respective column will be read with dtype numpy.float64, which may cause problems down the line as spec says this should be a string. Not a issue currently since all currently available priors require two parameters, but may lead to problems if that changes.

Locate the code that reads the objectivePriorParameters column in PEtab files and inspect how empty, single-value, and multi-value columns are converted. Add a regression check showing that empty or single-value parameters retain the string dtype required by the specification, then verify the existing parsing tests still pass.