Future warning on pandas frame.append

[cbworden]

Describe the bug
/Users/cbworden/miniconda/envs/shakemap/lib/python3.8/site-packages/impactutils/mapping/city.py:329: FutureWarning: The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
newdf = newdf.append(tdf)

[gferragu]

Just commenting to note this was discussed yesterday in the groundmotion processing meeting. No functionality issues there, just lots of warning output during map generation. It can be fixed by switching from append to concat here. The append command has been deprecated because it is a special case of concat, which can specify the axis to concatenate along. The fix would be something like newdf = newdf.concat(tdf, axis=0, join='outer'), but these are the default values in concat anyway. I know @emthompson-usgs is refactoring the code, so this could be fixed in that or as a separate quick PR.