Solution: add default_diffusion_coeff

[rkingsbury]

Summary

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+.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (50b11a6) 80.30% compared to head (a62ea2c) 80.17%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #101 +/- ## ========================================== - Coverage 80.30% 80.17% -0.13% ========================================== Files 10 10 Lines 1447 1448 +1 Branches 252 252 ========================================== - Hits 1162 1161 -1 - Misses 250 251 +1 - Partials 35 36 +1 ```

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