Update releases branch with changes from  19/06 discussion

See merge request !36

set OMP_NUM_THREADS for make checks

See merge request !32

Resolve "Default value for GCC_ARCH"

See merge request !34

conf-wrappers: set LIBS_ELPA FCFLAGS_ELPA

See merge request !31

check for availability of the fortran module for elpa_openmp before setting ELPA environment overrides

remove superflous '-L' option: lmod-spack already sets $LIBRARY_PATH, and the configure wrapper generates RPATH for linking

set fake-interpreter to /bin/sh in order to run checkbashisms and fix a bashism in all octopus configure wrappers

Resolve "Improve octopus configure wrappers"

See merge request !28

Resolve "[Docker] mpiexec not set"

See merge request !30

Resolve "Downgrade nfft to 3.2.4 in toolchains"

See merge request !29