Alleviate Pandas .ix indexing depreciation warning

[aerispaha]

Pandas is giving this warning due to a flavor of indexing set to change in future versions:

See the documentation here:
  http://pandas.pydata.org/pandas-docs/stable/indexing.html#ix-indexer-is-deprecated
    full_set = pd.concat([df1.ix[common_ids], df2.ix[common_ids]])
  C:\projects\swmmio\swmmio\version_control\inp.py:152: DeprecationWarning: 
  .ix is deprecated. Please use
  .loc for label based indexing or
  .iloc for positional indexing

Update swmmio to use the supported Pandas indexing pattern.