Fix review comments: add MultiGrid module with correctness and hygiene fixes

[Copilot]

Introduces the MultiGrid module (from PR #97) with all review-requested fixes applied before merging.

Fixes applied

• ProlongationBsplines.hpp

  • Added missing #include <tuple> (was relying on transitive includes)
  • Guarded the knot search loop in nonZerosInfoRow() against out-of-bounds access and unsafe decrement when the refined knot value is at/past the last knot
  • Hoisted nonZerosInfoRow() call out of the inner j loop — it only depends on i, not j
  • Replaced non_zero_alpha(j) linear indexing with explicit non_zero_alpha(0, j)

• MultiGridSolver.hpp

  • Replaced macro constants (#define MULTIGRID_*) with a scoped unnamed enum to avoid macro pollution in including translation units:

    // before
    #define MULTIGRID_minLvl 1
    #define MULTIGRID_cycleType 2
    
    // after
    enum : unsigned int { MULTIGRID_minLvl = 1, MULTIGRID_cycleType = 2, ... };

• Prolongation.hpp

  • Cached fine_glue.get_glue_matrix() and coarse_glue.get_glue_matrix() into local const variables to avoid repeated by-value returns and redundant sparse transposes

• Bembel/MultiGrid (module header)

  • Added trailing // BEMBEL_MULTIGRID_MODULE_ comment on #endif to match the convention used by all other module headers

• examples/LaplaceBeltramiMultiGrid.cpp

  • Added max_iters parameter (default 1000) to mmg() and guarded the do/while loop with iter < max_iters to prevent indefinite hangs on non-convergent setups

• examples/CMakeLists.txt

  • Added LaplaceBeltramiMultiGrid to CIFILES so it is built and run in CI

[jdoelz]

Ok