Update thermal model

[dan-mccabe]

• Fix bug when stop locations were outside of county boundaries, making no TMY data available (and looking for a nonexistent nan.csv file.) Solves Bug in thermal impacts for CTA: geometries do not match #49
• Refactor thermal_impacts.py into smaller functions
• Add ability to predict thermal load for median day (instead of just hottest and coldest days)
• Changes to output format to handle the additional weather scenario. Now instead of adding Winter_HVAC_Energy and Summer_HVAC_Energy columns to the predictions, we add scenario (e.g., median) and hvac_energy columns.