Bug fix in roughness unit conversion when using D-W

[kaklise]

Summary

The following PR addresses #443 regarding unit conversion of the roughness coefficient when using D-W headloss formula. This update only impacts the EpanetSimulator which requires the unit conversion when reading/writing the INP file. The WNTRSimulator does not support D-W headloss formula.

The original code did not convert roughness in _read_pipes and _write_pipes. The to_si and from_si conversion functions for roughness were also incorrect and updated.

If wn.options.hydraulic.headloss is changed from ['H-W', 'C-M'] to/from 'D-W', a UserWarning is printed to the screen. The units of headloss cannot be converted from unitless to/from length.

Tests and documentation

Tests were added to test_epanet_io.py. The mean absolute error in pressure values for Net3 is within 0.55 m when using H-W, C-M, and D-W with GPM and LPS.

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.

[coveralls]

coverage: 84.333% (+0.1%) from 84.227%
when pulling 9a05834 on kaklise:dw_units
into 576dc01 on USEPA:main.