Improve load unit calculation

[moritz-weber]

Currently, a default number of 10 load units is implemented for all components that don't define another value. This appears to be valid for power system components, but other components don't have load units at all.

This could be implemented with a condition on the COMPONENT-BUILDER-OPTIONS: COMPONENT-MODE: PSYS.
Cable ends have STACKING-LOAD: 0 defined. For transmission lines, it's defined in the calc block.
Split cables and tlines have 5 load units per subsystem. No matter where the calc block is located.