Generalise locating configuration files in 'settings'

[pjbriggs]

Generalises the locate_settings_file function in the settings module, so that it can operate with arbitrary config file names, search one or more environment variables specified by the calling subprogram, and use a custom search path also specified by the calling subprogram.

When the function is used from within the Settings class to search for the auto_process.ini file, then the name, the environment variable (AUTO_PROCESS_CONF) and search path (current directory, config directory and installation directory) are now provided expliciting within that class (rather than implicitly within the locate_settings_file function).

This update will allow the locate_settings_file function to be reused in other packages. It should also provide some additional flexibility if needed in future.