  We extend Korevaar-Schoen's theory of metric valued Sobolev maps to cover the
case of the source space being an RCD space. In this situation it appears that
no version of the `subpartition lemma' holds: to obtain both existence of the
limit of the approximated energies and the lower semicontinuity of the limit
energy we shall rely on:
  - the fact that such spaces are `strongly rectifiable' a notion which is
first-order in nature (as opposed to measure-contraction-like properties, which
are of second order). This fact is particularly useful in combination with
Kirchheim's metric differentiability theorem, as it allows to obtain an
approximate metric differentiability result which in turn quickly provides a
representation for the energy density,
  - the differential calculus developed by the first author which allows,
thanks to a representation formula for the energy that we prove here, to obtain
the desired lower semicontinuity from the closure of the abstract differential.
  When the target space is CAT(0) we can also identify the energy density as
the Hilbert-Schmidt norm of the differential, in line with the smooth
situation.
