Introduce enum structure in fe_quadrature

Sources/+phase_field/+fem/+quadrature/fe_quadrature.m

Add an enum structure for the element type (elem_type) as in https://gitlab.ethz.ch/compmech/GRIPHFiTH/-/blob/devel/Sources/+phase_field/StressState.m, but with < double instead of < uint8