With the new Qt5 C++11 syntax, private slots are not really necessary.
Replacing it with a normal private method reduces the generated code
from moc and also syntax issues are raised as compiler errors instead
of runtime asserts.
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.
refs #689
