GitLab

This feature helps to reduce memory per image. Can be enabled by
specifying `-nb xxx` at the command line (see QE manual).

* Use updated Docker images.
* Check numerical results and fail the CI if results don't match.
* For now pw.x is always run with 2 MPI processes. Using more
  processes leads to relatively small noises in the wavefunctions,
  which make a noticeable difference in Wstat, and even a larger
  difference in Wfreq. Why this is happening needs further
  investigation.
* Fix doc build with sphinx 3.5.0+.
  (See https://github.com/sphinx-doc/sphinx/issues/8885)