Validate SystemConfig

[lorenzleutgeb]

SystemConfig should implement a mechanism to validate all configuration parameters. This means checking configuration parameters passed as Strings more strictly (i.e. whether they are an element of some sane set) and the range of numeric parameters, etc.

[AntoniusW]

This comment is intended to collect (non-obvious) conditions that have to be met by the config:

• Handle cross-dependencies (see Handle cross-dependencies in SystemConfiguration #303).
• After merging (Phase saving, restarts, and heuristic performance logs #202), phase saving may be enabled but is only supported by a few heuristics.