Added
- `Solution`: new kwarg `default_diffusion_coeff` which allows the user to specify a value to use
when a species diffusion coefficient is missing from the database. By default, the value for NaCl
salt (1.61e-9 m2/s) is used. This is important for conductivity and transport number calculations,
which perform weighted summations of diffusion coefficients over every species in the solution.
Previously, species with missing diffusion coefficients would be excluded from such calculations,
possibly resulting in inaccuracies, particularly after calling `equilibrate()`, which often
generates charged complexes such as NaSO4- or MgCl+.
Fixed
- `Solution.__add__`: Bugfix in the addition operation `+` that could cause problems with
child classes (i.e., classes that inherit from `Solution`) to work improperly
Changed
- Removed deprecated `pkg_resources` import in favor of `importlib.resources`