Add Gay-Berne interaction with tunable radial width

[abhispace27]

Fixes #

Description of changes:

This pull request adds a modified Gay-Berne non-bonded interaction with an
independently tunable radial width.

In the original Gay-Berne interaction, sig controls both the
orientation-dependent position of the potential minimum and the radial width
of the Lennard-Jones-like well. The new gay_berne_width interaction introduces
an additional parameter, wid, so that:

• sig retains control of the orientation-dependent minimum position;
• wid controls the radial width and local stiffness of the well.

The modified radial coordinate is

[
x_w = 2^{1/6} + \frac{r-r_{\min}}{\mathrm{wid}},
]

with

[
r_{\min} = s + \left(2^{1/6}-1\right)\mathrm{sig},
]

where (s) is the orientation-dependent Gay-Berne contact distance.

Setting wid = sig recovers the radial form of the existing ESPResSo
Gay-Berne interaction.

The implementation includes:

• force, torque, and energy kernels;
• director-based and quaternion-based overloads;
• cutoff and active-pair-mask integration;
• pressure/virial force integration;
• C++ script-interface registration;
• a Python interface exposed as
  system.non_bonded_inter[type1, type2].gay_berne_width;
• the compile-time feature GAY_BERNE_WIDTH, which implies ROTATION.

The code builds successfully locally with GAY_BERNE_WIDTH enabled.