obvious repeat on define diffusion particles and material

[Xiangyu-Hu]

The two lines shows that

SPHinXsys/tests/2d_examples/test_0d_regression_test/regression_test.cpp

Line 112 in 751280b

using DiffusionParticles = DiffusionReactionParticles<SolidParticles, DiffusionMaterial>;

and

SPHinXsys/tests/2d_examples/test_0d_regression_test/regression_test.cpp

Line 201 in 751280b

diffusion_body.defineParticlesAndMaterial<DiffusionParticles, DiffusionMaterial>();

repeat the use of diffusion material.

[Xiangyu-Hu]

I will revise this by: 1. using single type for all particles, 2. the variables will be registered in diffusion-reaction material or in diffusion reaction methods.