Refactoring continued

- reworked return periods (only changed via the property, not the metrics methods)
- removed npv boolean, risk is now always computed with risk discount if present
- reworked caching (results are now correctly cached)
- reworked computation related properties (now more tightly connected to
computings objects
- removed `time_points` from computing object as not usable
- clearer names

Author: spjuhel