Fix problem with duplicated (implicitly shared) lists.
Fix problem with duplicated (implicitly shared) lists.

The initialization with empty lists actually duplicates the initial list
everywhere. Instead, just create new lists and append them as necessary.

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