Allow relative file locations

[mnblonsky]

When file paths are not absolute, we currently look for the file in the OCHRE/defaults folder, which isn't always useful. It's probably best to assume the user specifies a relative location from their cwd. A few things to change here:

• Don't add default_input_path to HPXML, schedule, and maybe weather files
• For other files, check if relative files exist before adding default_input_path. If file not found, use relative file in the error
• Remove input_path parameter to Dwelling (maybe Simulator too)