Environment variables in config files

[gonzaponte]

The current code to read config files only interprets environment variables in two cases:

• When a file is included in the config file through the include function
• For the input and output files

Moreover, the two cases are handled by two separate entities. While the configure.py module manages the first, the city decorator in components.py takes care of the second. I propose we

• unify the handling of environment variables in configure.py
• apply environment variable expansion to all relevant parameters in the config file (not only input and output files).