feat(THM): Add mass balance equation type

In the current THM implementation, the flow equation is based on the volume‑balance formulation, which reduces solution accuracy when liquid density depends on either pressure or temperature or both. This MR introduces functionality that makes the mass‑balance equation mandatory whenever a non‑constant liquid density is used. To support this, more detailed local Jacobian evaluations have been added.

The changes introduce a new optional input tag:

<equation_balance_type>mass/volume</equation_balance_type>

into <processes>\<process>.

The volume‑balance formulation is used as default if that tag is missing. A fatal error will be issued if volume‑balance formulation is used while liquid density model is not constant.

1. Tests covering your feature were added? with the existing benchmarks.
2. Any new feature or behaviour change was documented?

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