Provenance and licensing of the database folder

[JaroCamphuijsen]

The mibiremo/database folder in this repository holds four datasets. It is not clear where this data originates from, under what license this data is published, whether we are allowed to republish it, etc. It would be good to add this information to the README in that folder and specify data licenses for each of the datasets.

Also perhaps remove files that are not needed.

[matteomasi]

One strategy can be to use the phreeqc.dat file provided in the phreeqcrm pip package and use a script to merge the newly implemented biological reactions to produce a new on-the-fly database file that can be used in the simulations. This avoids redistributing the database files.