Setting competition.simulationBaseTime through config produces a warning

[jecollins]

When the server starts, one of its first activities is to run the configuration process. Many of the config variables are in Competition, and one of the consistently posts a warning when the server starts, as

47120 WARN  config.Configurator: Type mismatch, class org.powertac.common.Competition: cannot use default value (org.joda.time.Instant) for  (java.lang.String)

This is happening because the setter withSimulationBaseTime() takes a String, while the getter returns an Instant. The time appears to be set correctly, but this should be fixed.