Geochemistry: fully gold and document the equilibrium tests

[WilkAndy]

Reason

Current Gold in the equilibrium test files is very limited - I mean, the tests only check a very few things, and more output needs to be checked. And the current documentation does not discuss the MOOSE results in comparison with GeochemistsWorkbench. The reason for this is that the current MOOSE implementation does not yet include temperature dependence properly, so the MOOSE results aren't expected to be perfect: no point testing or commenting on imperfect output! See #14982.

Design

All the tests in the following directories need more extensive Golding:

geochemistry/test/tests/equilibrium_models
geochemistry/test/tests/redox_disequilibrium
geochemistry/test/tests/solubilities_and_activities
geochemistry/test/tests/solubilities_and_activities

The following documentation needs to include a comparison of the MOOSE results with the GeochemistWorkbench results:

amazon.md
gypsum.md
microbial_redox.md
morro.md
red_sea.md
seawater.md
sebkhat.md
selenate.md
surface_complexation.md

This will be possible when #14982 is merged, and temperature dependence properly incorporated into the Solver. So I won't be addressing this issue for a few weeks.

Impact

More thorough testing and documentation

[WilkAndy]

Completed by the above commits