Add 1D Euler-Bernoulli beam solver

[nikoscham]

• Add a 1D Euler-Bernoulli beam bending solver (eulerBernoulliBeamScript) using cubic
  Hermite (C¹) finite elements, so both deflection w and slope theta = dw/dx are
  represented as nodal DOFs. The fourth-order analogue of the existing scalar 1D solvers
  (heatConductionScript, generalFormPDEScript).
• Add hermiteCubic support to basisFunctions.js (shape functions + 2nd ksi-derivative)
  and a matching 6-point Gauss rule to numericalIntegration.js. This is general C¹
  interpolation infrastructure, not beam-specific, eulerBernoulliBeam.js is just its
  first consumer, and it should be directly reusable for future 4th-order models (e.g.
  Kirchhoff plate bending).
• Add a worked example reproducing the "Bending of a Beam" problem from J.N. Reddy,
  An Introduction to the Finite Element Method, 3rd ed. (FEM1D reference program,
  Chapter 7): a beam clamped at one end, supported by a roller at midspan and a linear
  spring at the free end, carrying a distributed load, a point moment, and a point load.

[nikoscham]

This task has been implemented by @ferrari212