add units for PINN forward examples

[chaoming0625]

This pull request includes multiple changes across various files to improve the documentation and functionality of the project. The most significant changes involve the removal of markdown cells from several Jupyter notebooks, the addition of unit conversions in Python scripts, and the inclusion of new unit examples in the documentation.

Documentation Updates:

• docs/examples-pinn-forward/Beltrami_flow.ipynb: Removed markdown cells explaining the three-dimensional Beltrami Flow problem.
• docs/examples-pinn-forward/Euler_beam.ipynb: Removed markdown cells explaining the Euler beam problem setup.
• docs/examples-pinn-forward/Helmholtz_Dirichlet_2d.ipynb: Removed markdown cells explaining the Helmholtz equation over a 2D square domain.
• docs/examples-pinn-forward/diffusion_1d.ipynb: Removed markdown cells explaining the diffusion equation.

Code Enhancements:

• docs/examples-pinn-forward/Beltrami_flow.py: Added unit conversions for space, speed, time, and pressure. Updated the PDE and initial/boundary conditions functions to use these units. [1] [2] [3] [4] [5]
• docs/examples-pinn-forward/Euler_beam.py: Added unit conversions for displacement, length, modulus of elasticity, moment of inertia, and load. Updated the PDE and boundary conditions functions to use these units.

Minor Changes:

• docs/examples-pinn-forward/Klein_Gordon.ipynb: Added an empty code cell.
• docs/examples-pinn-forward/Kovasznay_flow.ipynb: Added an empty code cell.
• docs/examples-pinn-forward/Kovasznay_flow.py: Added a blank line after imports and removed commented-out return statement in the pde function. [1] [2]
• docs/examples-pinn-forward/elasticity_plate.py: Fixed formatting issues and removed commented-out code in the pde function. [1] [2] [3] [4]

Documentation Structure:

• docs/unit-examples-forward.rst: Added new unit examples for Beltrami flow, diffusion 1D, Euler beam, and Helmholtz Dirichlet 2D to the documentation.