Add a required<T>(para, key) helper to cut per-material parameter boilerplate

[petlenz]

Each material spends roughly 40 lines declaring parameters:

para.template insert<indices>("indices").template add<is_required>();

and again reading them with base::template get_parameter<T>("k").

A free helper required<T>(para, "k") (and optional<T>(para, "k", default)) was tried during review and works; using base::add_output does not, because of the dependent-name lookup.

Purely additive: existing materials keep compiling, new ones get shorter, and the .template noise disappears from the code a material author has to write. It is also the natural place to attach check_data_type and range<> once, see the parameter-diagnostics issue.

All references are to main at 91bd26f.