Enable different configuration locations

[gnn]

Relying on the configuration file being in the home directory makes some things (e.g. behaviour on missing sections) difficult to test as it's impossible to know what's in the configuration file at test time. Being able to specify a differen't location (or something similar) would make testing things easier (and could have other benefits).
Fortunately there are a few ways of implementing this without breaking backwards compatibility:

• implement a way to override configuration settings from within python code,
• use a default (documented) set of locations to search for configuration files.