Homogenized tangent for homogeneous material is nan

[strinner213]

For the trivial cases where all materials in a composite have same isotropic properties (for the example below: diffusion coefficient of 1) the effective response should also be the same (here: identity matrix). However, the computed homogenized tangent is nan.

Relevant parts of the input-json to reproduce:

    "problem_type": "thermal",
    "matmodel": "GBDiffusion",
    "material_properties":{
        "GB_unformity": true,
        "D_bulk": 1.0,
        "D_perp": 1.0,
        "D_par": 1.0
    },

    "method": "cg",
    "error_parameters":{
        "measure": "Linfinity",
        "type": "absolute",
        "tolerance": 1e-10
    },

    "n_it": 1000,
    "macroscale_loading":   [
                                [[0,0,0]]
                            ],    

    "results": ["homogenized_tangent"]

Computation of stress average with uniaxial macroscopic loading works correctly.