Add pseudospectral grid setup utilities and integration/plotting example

[haakoek]

Summary:

• Adds reusable setup helpers for pseudospectral grids.
• Exposes setup helpers from the pseudospectral_grids package namespace.
• Adds an example that builds multiple grids, plots $f(x) = e^{-x^2}$, and evaluates quadrature integrals against the exact Gaussian integral sqrt(pi).

What changed:

• Added grid_lib/pseudospectral_grids/setup_grids.py:
  • setup_grid for sinc, gll, and femdvr grids
  • setup_femdvr_uniform convenience constructor
  • mapping and element-class support for flexible construction
• Updated grid_lib/pseudospectral_grids/init.py:
  • exports Rational_map
  • exports setup_grid and setup_femdvr_uniform from setup_grids
• Added examples/setup_different_grids.py:
  • configures Sinc DVR, GLL, FEM-DVR, and uniform FEM-DVR grids
  • plots $f(x) = e^{-x^2}$ on each grid in one multi-subplot figure
  • computes quadrature via $sum_i w_i f(x_i)$
  • compares numerical values to the exact full-domain integral $\int_{-\infty}^{\infty} e^{-x^2}dx = \sqrt{\pi}$