Add Kronecker sampler as well as tests and a demonstrating notebook

[Samuel-Vangu]

Adds a Kronecker sampling class for integration on (hyper)cubic grids,
following up on the discussion in #322.

What's included

• src/grid/kronecker.py: Kronecker(n_points, dimension, seed=None, randomize=True, origin=None, axes=None). Generates the sequence
  $x_i = {i \cdot \sqrt{p_j}}$ using the square roots of the first $d$ primes
  (via sympy.sieve) as irrational multipliers; randomize=False gives the
  deterministic sequence (whose first point is always the origin),
  randomize=True applies a random Cranley-Patterson shift. No power-of-2
  or other special constraint on n_points is required. Supports mapping
  onto an arbitrary parallelepiped via origin/axes.
• src/grid/tests/test_kronecker.py: tests covering validation, weights,
  domain mapping, integration accuracy, reproducibility, and properties
  specific to Kronecker sequences (construction verified directly against
  an independent calculation with sqrt(2)/sqrt(3)/sqrt(5), origin as first
  point when unrandomized, and the prefix/nesting property -- an N-point
  design is always an exact prefix of a larger design with the same seed).

• An example notebook in examples/