New sorption formulation

[Manangka]

MODFLOW supports modeling of sorption in the transport model using Isotherms.
Isotherms are non-linear functions that establishes a relation between the sorbed concentration and the solute concentration in the groundwater.
$f_{sorption} =-f_m \rho_b \frac{\partial \left(S_w \overline{C}\right)}{\partial t}$

Currently MODFLOW is using the midpoint point rule to discretize the sorption term.
$f_{sorption} = -f_m \rho_b\left(\left(S_w \frac{\partial \overline{C}}{\partial C}\right)^{n+0.5} \frac{C^{n+1} - C^{n}}{\Delta t} +\overline{C}^{n+0.5}\frac{S_w^{n+1} - S_w^{n}}{\Delta t}\right)$

However some investigation showed that this formulation isn't mass conservative.
A test (test_gwt_mst08) has been added to this PR that demonstrates this.

This PR reformulates the sorption term such that it becomes mass conservative. It does this by linearizing the $S_w \overline{C}$ term and freezing the jacobian at the last picard iteration.
$f_{sorption}=-f_m \rho_b \frac{S_w^{\left(k\right)}\left(\frac{\partial \overline{C}}{\partial C}\right)^{\left(k\right)} \left(C^{n+1} - C^{\left(k\right)}\right) + \overline{C}^{\left(k\right)}S_w^{n+1} - \left(S_w \overline{C}\right)^{n}}{\Delta t}$

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Further a modification to the Freundlich Isotherm is added to improve stability.
The isotherm becomes problematic for values of a that are smaller than 0.
For those values the derivative goes to infinity causing convergence issues.

The Freundlich Isotherm is given by:
$\overline{C} = K_f C^a$

with its derivative
$\frac{\partial \overline{C}}{\partial C} = a K_f C^{a-1}$

To improve stability a small epsilon is added to keep the derivative finite.
$\overline{C} = K_f \left(C + \epsilon\right)^{a} - K_f \epsilon^{a}$

Note that in the equation above the value of $\overline{C}$ at 0 equals the original Freundlich equation

The corresponding derivative now becomes finite:
$\frac{\partial \overline{C}}{\partial C} = a K_f \left(C + \epsilon\right)^{a - 1}$

Checklist of items for pull request

• Replaced section above with description of pull request
• Closed issue #xxxx
• Referenced issue or pull request #xxxx
• Added new test or modified an existing test
• Ran ruff on new and modified python scripts in .doc, autotests, doc, distribution, pymake, and utils subdirectories.
• Formatted new and modified Fortran source files with fprettify
• Added doxygen comments to new and modified procedures
• Updated meson files, makefiles, and Visual Studio project files for new source files
• Updated definition files
• Updated develop.toml with a plain-language description of the bug fix, change, feature; required for changes that may affect users
• Updated input and output guide
• Removed checklist items not relevant to this pull request

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