Previously they were returned by value, but I do not remember any
actual reason for that.  Return by const reference leads to a
measurable speed increase when computing the derivatives of
projection-based FE onto the sphere.  It also fixed some
undefined behavior in that code, which took const-references
of the returned temporaries.

Returning the coefficients by const reference is more difficult
for product manifolds, because previously the interpolation rules
stored the coefficients separately for each factor space.
With this patch, these rules now store the coefficients twice:
once in the old separate format, and once as needed for returning
them by reference.  Interpolation rules are not meant to exist
in large numbers, and therefore I do not think that this
extra space consumption matters.