Add inp coverage for new SWMM5.2 Features: STREETS, INLETS, INLET_USAGE

[aerispaha]

Summary

This adds coverage for the new model objects introduced in SWMM5.2, as described here. Doctests are also provided for each new inp section showing how to get/set data in the STREETS, INLETS, and INLET_USAGE sections.

Usage

model = swmmio.Model('path-to-model-file.inp')

# get the streets dataframe 
model.inp.streets

# get inlets dataframe 
model.inp.inlets

# get inlets_usage dataframe 
model.inp.inlet_usage

Closes #180
Makes progress on #57

[bemcdonnell]

@aerispaha you're from da model.inp.streets!