Rename CI_* modules to avoid confusion w/ continuous integration

[sadielbartholomew]

Since 'CI' is, to developers, generally interpreted as standing for 'continuous integration', our pair of modules using 'CI' to stand for 'carbon intensity' ( CI_api_interface.py and CI_api_query.py) have names which can easily be misinterpreted as relating to continuous integration, so we should rename these to avoid the acronym. I suggest intensity instead of CI directly in-place, e.g. intensity_api_interface.py, since intensity implies carbon intensity in the context (in preference to carbon_intensity in full, which would make the file names quite long, too long IMO).