Unification of materials with interpolations

**Current status**: Active

**Main classes/functions**: HomogenizedMicrostructureInterpolator, DensityBasedMaterial, MaterialPhaseFieldAnalytic

**Outline of the solution**
- [ ] Create general elastic material multiplied by any interpolation function
- [ ] Restructure the material interpolations to unify nomenclature

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*Assigned to:* @gervilquin 
*Wingman:*
