template<>
action<push_matvar> struct
Add matched outvar based on matvar into vector of outvars
Push outvar based on matvar: use depvar char as OutVar::var, OutVar::name = "" by default. Matvar-based outvars are similar to depvar-base outvars, in that the OutVar has empty name and the OutVar::var is a depvar, but instead of having the user try to guess the field id, the grammar accepts a physics label (accepted multimatvars) and a material index, which are then converted to a depvar + field index.