"split_trips" sometimes does not work out for unknown reason

[ywnch]

Just run the function again if "plot_tsd" does not look right at the moment, as shown below. A permanent fix is required later on.

Also fix the two caveats:

1. See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
   df['VehDistAlongRoute'] = df['CallDistanceAlongRoute'] - df['DistanceFromCall']
   /Users/Yuwen/Dropbox/BusSimulator/buskit/buskit/busdata.py:244: SettingWithCopyWarning:
   A value is trying to be set on a copy of a slice from a DataFrame.
   Try using .loc[row_indexer,col_indexer] = value instead

2. See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
   .dt.tz_convert('America/New_York')
   /Users/Yuwen/anaconda/lib/python3.6/site-packages/pandas/core/computation/expressions.py:179: UserWarning: evaluating in Python space because the '*' operator is not supported by numexpr for the bool dtype, use '&' instead
   .format(op=op_str, alt_op=unsupported[op_str]))