Extending point eval handler

[termi-official]

Currently we just have an allocating version for evaluate_at_points and the internal function evaluate_at_points! which requires knowledge about the internals to use. We should widen the user-facing portion by adding the following functions

• evaluate_at_points!(values::AbstractVector, ph::PointEvalHandler, proj::L2Projector, dof_vals::AbstractVector)
• evaluate_at_points!(values::AbstractVector, ph::PointEvalHandler, dh::DofHandler, dof_vals::AbstractVector)
• evaluate_at_points!(values::AbstractVector, ph::PointEvalHandler,dh::DofHandler, dof_vals::AbstractVector, fname::Symbol)

Furthermore we might want to restrict the evaluation to subdomains, which requires a little bit of refactoring for the internals.