Use GeometryMapping in BCValues

The `GeometryMapping` in #764 should be reusable in `BCValues`. To avoid littering that PR with even more stuff, I'm leaving some preliminary and untested code here as a reference.

To be done in a separate PR after merging 764...

```julia
"""
BCValues(func_interpol::Interpolation, geom_interpol::Interpolation, boundary_type::Union{Type{<:BoundaryIndex}})

`BCValues` stores the shape values at all faces/edges/vertices (depending on `boundary_type`) for the geometric interpolation (`geom_interpol`),
for each dof-position determined by the `func_interpol`. Used mainly by the `ConstraintHandler`.
"""
struct BCValues{V_GM, FQR} <: AbstractFaceValues
geo_mapping::V_GM # AbstractVector{GeometryMapping}
fqr::FQR # FaceQuadratureRule
current_face::ScalarWrapper{Int}
end

BCValues(func_interpol::Interpolation, geom_interpol::Interpolation, boundary_type::Type{<:BoundaryIndex} = Ferrite.FaceIndex) =
BCValues(Float64, func_interpol, geom_interpol, boundary_type)

function BCValues(::Type{T}, func_interpol::Interpolation{refshape}, geom_interpol::Interpolation{refshape}, boundary_type::Type{<:BoundaryIndex} = Ferrite.FaceIndex) where {T,dim,refshape <: AbstractRefShape{dim}}
# set up quadrature rules for each boundary entity with dof-positions
# (determined by func_interpol) as the quadrature points
interpolation_coords = reference_coordinates(func_interpol)

qrs = QuadratureRule{refshape,T,dim}[]
for boundarydofs in dirichlet_boundarydof_indices(boundary_type)(func_interpol)
dofcoords = Vec{dim,T}[]
for boundarydof in boundarydofs
push!(dofcoords, interpolation_coords[boundarydof])
end
qrf = QuadratureRule{refshape,T}(fill(T(NaN), length(dofcoords)), dofcoords) # weights will not be used
push!(qrs, qrf)
end
fqr = FaceQuadratureRule(qrs)
geo_mapping = [GeometryMapping{0}(T, geom_interpol, qr) for qr in qrs]

BCValues(geo_mapping, fqr, ScalarWrapper(1))
end

nfaces(bcv::BCValues) = length(bcv.geo_mapping)
getcurrentface(bcv) = bcv.current_face[]

function set_current_face!(bcv::BCValues, face_nr::Int)
# Checking face_nr before setting current_face allows us to use @inbounds
# when indexing by getcurrentface(fv) in other places!
checkbounds(Bool, 1:nfaces(bcv), face_nr) || throw(ArgumentError("Face index out of range."))
bcv.current_face[] = face_nr
end

getnquadpoints(bcv::BCValues) = @inbounds getnquadpoints(bcv.fqr, getcurrentface(bcv))
```