Add option to change the log gf cut applied by carsus to the documentation

[afloers]

Is your feature request related to a problem? Please describe.
Carsus allows the user to create their own TARDIS atomic data file (see e.g. https://tardis-sn.github.io/carsus/quickstart.html). Before dumping the file to hdf5, a cut on the quantity log(g f) (with g being the statistical weight of the level and f the oscillator strength of the transition) is applied. Currently, the log(g f) cut is set to -3, but can be changed by the user when creating a TARDISAtomData object.

carsus/io/output/base.py

Describe the solution you'd like
The value of the log(g f) cut and that it can be adjusted should be stated in the documentation.