Implementation of changing lambdas and mus over subdomain interface

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```cpp
void LinElastSolver::BuildDomainOperators()
{
// SanityCheckOnCoeffs();
as.SetSize(numSub);

for (int m = 0; m < numSub; m++)
{
as[m] = new BilinearForm(fes[m]);
as[m]->AddDomainIntegrator(new ElasticityIntegrator(*(lambda_c[m]), *(mu_c[m])));

if (full_dg)
{
as[m]->AddInteriorFaceIntegrator(
new DGElasticityIntegrator(*(lambda_c[m]), *(mu_c[m]), alpha, kappa));
}
}

a_itf = new InterfaceForm(meshes, fes, topol_handler);
a_itf->AddIntefaceIntegrator(new InterfaceDGElasticityIntegrator(lambda_c[0], mu_c[0], alpha, kappa));
}
```

How should the last line be handled?

Start with LinElastSolver::BuildDomainOperators and inspect InterfaceDGElasticityIntegrator alongside ElasticityIntegrator and DGElasticityIntegrator. Determine how changing lambda_c and mu_c across subdomains should be represented at the interface; the work is done when the final interface-integrator construction handles those coefficients consistently.

Written by the indexing model from the issue text.