CPluginManagerSimulator::createListener returns nullptr if no
listener could be created, e.g. because the plugin could not be loaded
due to missing dependencies. Handle this scenario properly instead of
raising an assert.
refs #669
Change MetarSet to MetarList:
CMetarSet was implemented as a collection. This only makes sense for values which have one member variable only or hardly ever change. METARs often change and therefore a collection didn’t make much sense.
Additional profiling showed that a sequence has better algorithmic performance.
