Fixes renwick shield generators incorrectly using power

- Shield generators now correctly use power according to actual field strength.
- Minimal shield dissipation rate now corresponds with 1 renwick field (changed from 10, which technically caused shields to always use max upkeep power ignoring field strength completely)

This issue made it impossible to create larger fields simply due to massive and illogical power usage.  So you can now actually create <b>weak</b> shield around the station. However don't expect these shields to stop everything you throw at them (for example, meteor hit breaches fields up to ~2 renwicks, and strong explosions can take as much as 6 renwicks.