Large refactoring of the IO mixins (PIMixin and CSVMixin for both
SimulationProblems and OptimizationProblems.

We add a general DataStoreAccessor class extended by OptimizationProblem
and SimulationProblem, which is used to store all data read by the IO
mixins in a DataStore object. Higher lever mixins like PIMixin/CSVMixin
do not store their data themselves anymore. Of course users are still
free to override get_timeseries/set_timeseries and store data themselves
in their own mixins.

For both simulation and optimization we also add an IOMixin class for
which contains methods that were previously duplicated in their
respective PIMixin and CSVMixin classes. There is still quite a bit of
duplicate code between the IOMixin for optimization and the IOMixin for
simulation (and similarly for the respective PIMixins and CSVMixins),
but that is something a future commit will address.