Make update of code stats incremental

[oruebel]

Currently, updating the code stats requires deleting and recomputing all results (e.g. CLOC), which takes a lot of time and also requires a large number of API calls for GitHub and Codecov. To avoid this and make it easier for us to deploy the site updates should be incremental, i.e., only computing results starting from the last date/commit that has been cached.