Non-allocating point eval handler evaluations

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.