Built new custom gaussian potential which is anisotropic

[abhispace27]

Fixes #

Description of changes:

• Added a new anisotropic Gaussian non-bonded interaction.
• The potential has independent width parameters in the x, y, and z directions:
  ( U = \epsilon \exp[-0.5(dx^2/\sigma_x^2 + dy^2/\sigma_y^2 + dz^2/\sigma_z^2)] )
• Added C++ core implementation for energy and force calculation.
• Added Python script-interface exposure through:
  system.non_bonded_inter[type1, type2].gaussian_aniso
• Added documentation for the new interaction.
• Added a simple two-particle test script to verify energy and force behavior.

Notes:

• Positive eps gives a repulsive Gaussian interaction.
• Negative eps gives an attractive Gaussian interaction.
• The interaction uses sig_x, sig_y, sig_z, and cutoff as parameters.

Testing:

• Built ESPResSo successfully with GAUSSIAN_ANISO enabled.
• Ran a two-particle test comparing measured energy and forces against the analytical expression.
• Verified both repulsive and attractive cases.