Improve log reading

[janosg]

Problem

• The functions in read_log.py still assume that params are a DataFrame
• These functions are not well documented
• Some tests of logging related functions rely on stored databases. This makes it very hard to adjust the logging code in general

Solution

• Rewrite all tests that require databases such that they generate their own databases
• Adjust the functions in read_log.py to work for general pytrees. Functions that return DataFrames where rows are value columns of parameters should probably just return lists of params now.
• Write documentation.