update README.md.

Signed-off-by: 刘明宏 <liuminghong@mail.sdu.edu.cn>

README.md

@@ -13,9 +13,12 @@ MATLAB Version: R2023a (or later)
 Memory: The actual memory usage depends on model size and number of frequencies. 
 #### 2.2 Required Software
 MATLAB (R2023a or later): The core environment in which em3d-MT operates.
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 em3d-MT relies on several external tools and libraries to facilitate 3D geological modeling and mesh generation. The following software is required for proper functionality:
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 GeoMesh
 For 3D geological modeling and mesh generation, GeoMesh is used. This is an open-source tool specifically designed for electromagnetic (EM) modeling, based on the COMSOL Multiphysics and MATLAB interface (Liu et al., 2024). GeoMesh generates unstructured tetrahedral meshes, which are ideal for representing complex subsurface structures in the forward modeling process. You can find the tool at https://gitee.com/sduem/geomesh
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 Additionally, em3d-MT supports two types of direct solvers for solving the finite element system:
 Panua-Pardiso:
 An optimized direct solver used for solving large sparse linear systems. It offers high performance and memory efficiency. More information can be found at https://panua.ch/pardiso/