MNT: Simplify exporting of instances.
MNT: Simplify exporting of instances.

Use sets instead of lists and manual iteration; extend can take an
iterable.

ahaberlie/MetPy,dopplershift/MetPy,ShawnMurd/MetPy,Unidata/MetPy,Unidata/MetPy,ahaberlie/MetPy,dopplershift/MetPy,jrleeman/MetPy,jrleeman/MetPy,ahill818/MetPy