Add environment variables and/or config file re file finding

[chipbarnaby]

Goal: allow simpler runtime setups by supporting configuration settings such as search paths for weather files, include files, and equipment data files (when 205 or HPWH data files are implemented).

Config settings could be in environment variables or a config ("ini") file or both.

Any related command line switches should have higher precedence.

Later thought (Feb. 2025): generalize include file search to apply to all external files. That is, don't make a granular/complex scheme when a single multi-directory list of search paths would handle most use cases.