Better handling of rounding-mode context for decimal calculations in table-generator

[PhilippWendler]

In table-generator, we use the rounding mode ROUND_HALF_UP. We set this globally in columns.py, which creates problems if someone imports table-generator, and for #917.
We also do not want to specify the rounding mode for every single operation because it can easily be forgotten. But we can set the rounding mode using a local context by wrapping with decimal.localcontext() as context: around all entry points. That would avoid the change of the global state and still be less risk. We just have to check whether this automatically works for computations done in worker processes with the multiprocessing module.