Fix bug on GIS IO caused by varying index names

[kbonney]

Summary

Below produces a minimal working example of the bug:
from os.path import join
import wntr

wn_path = "io.inp"
wn = wntr.network.WaterNetworkModel(wn_path)
wn_gis = wn.to_gis()
wn_gis.junctions.index.name = "my_index"
wn2 = wntr.network.from_gis(wn_gis)

WNTR currently expects wn_gis dataframes to all have indexes that are all named None. If custom names are present, the creating of the WN fails. This PR provides a small change to the IO code which accounts for the possibility of named indexes.

Tests and documentation

No tests or docs added.

Acknowledgement

By contributing to this software project, I acknowledge that I have reviewed the software quality assurance guidelines and that my contributions are submitted under the Revised BSD License.

[kbonney]

Closing in favor of #414

[kbonney]

Reopening since #414 was closed by user